Objective To focus on the possible mechanism about the spermicidal effect of methanolic extract of leaves of Cestrum parqui (Solanacea) on human spermatozoa. Methods Sperm motility and viability were noted according to the guideline of WHO. The morphological changes in chromatin materials and in plasma membrane at the head part of the spermatozoa were assessed under transmission electron microscopy (TEM). DNA fragmentation index (DFI) of spermatozoa in percentage was assessed by terminal deoxynucleotidyl transferase mediated dUTP nick end labelling (TUNEL) using kits. For confirmation of DNA breakage, gel electrophoresis of DNA was conducted using 1.2 % agarose gel. Results Sperm viability and motility were both decreased in dose-dependent manner from 50 to 300 mg/ml of methanolic extract in respect to the control. Loss of viability and motility both were noted 100% at the dose of 300 mg/ml within 5 min. From the microphotography it has been revealed that chromatin condensation at the dose of 200 mg/ml is more than the control without any noticeable alteration in plasma membrane. In contrast, chromatin decondensation has been noticed at the dose of 300 mg/ml along with a remarkable disruption in plasma membrane. Analysis of DNA damage by TUNEL revealed a significant elevation (P<0.01) in the level of % DFI at the dose of 200 mg/ml (moderate dose) after 17 h in respect to the control. But at the dose of 300 mg/ml, no significant variation was observed in this parameter upto 24 h of exposure though a significant variation (P<0.01) was noted after 48 h of exposure compared with the control. Gel electrophoresis of DNA also followed the same type of results i.e. 200 mg/ml affected DNA integrity but other doses were ineffective in this concern. Conclusion The extract of Cestrum parqui at moderate dose exhibited spermicidal activity by inducing apoptotic pathway but at the high dose it caused necrosis. Objective To focus on the possible mechanism about the spermicidal effect of methanolic extract of leaves of Cestrum parqui (Solanacea) on human spermatozoa. Methods Sperm motility and viability were noted according to the guideline of WHO. The morphological changes in chromatin materials and in plasma membrane at the head part of the spermatozoa were assessed under transmission electron microscopy (TEM). DNA fragmentation index (DFI) of spermatozoa in percentage was assessed by terminal deoxynucleotidyl transferase mediated dUTP nick end labeling (TUNEL) using kits. For confirmation of DNA breakage, gel electrophoresis of DNA was conducted using 1.2% agarose gel. Results Sperm viability and motility were both decreased in a dose-dependent manner from 50 to 300 mg / ml of methanolic extract in respect to the control. Loss of viability and motility both selected noted 100 % at the dose of 300 mg / ml within 5 min. From the microphotography it has been revealed that chromatin condensation at the dose of 200 mg / ml is more than the control without any noticeable alteration in plasma membrane. In contrast, chromatin decondensation has been noticed at the dose of 300 mg / ml along with a remarkable disruption in plasma membrane. Analysis of DNA damage by TUNEL DF at the dose of 200 mg / ml (moderate dose) after 17 h in respect to the control. But at the dose of 300 mg / ml, no significant variation was (P <0.01) in the level of% DFI at the dose of 200 mg / observed in this parameter up to 24 h of exposure though a significant variation (P <0.01) was noted after 48 h of exposure compared with the control. Gel electrophoresis of DNA followed the same type of results ie 200 mg / ml affected DNA integrity but Conclusion The extract of Cestrum parqui at moderate dose exhibited spermicidal activity by inducing apoptotic pathway but at the high dose it caused necrosis.