Radical production in the ozonolysis of propene in air was monitored directly by a peroxy radical chemical amplification (PERCA) instrument at room temperature (298±2 K) and atmospheric pressure (1×105 Pa). The ozonolysis reactions were conducted in a flow tube under pseudo-first-order conditions for ozone. The decay in ozone was calculated based on reaction time tr and effective rate constant keff (keff = k1[C3H6]0)) for the ozone-propene reaction. The total radical yields relative to consumed ozone were determined to be 0.97±0.17 (σ). A box model was used to simulate radical production in the ozone-propene reaction. The model-derived radical yields agree well with those from experiment within the uncertainty of the two methods. In addition, the yields for OH, HO2 and CH3O2 were obtained from the measured total radical yields and radical partitioning derived from the box model. The yield is 0.39±0.08 (σ) for OH, 0.19±0.04 (σ) for HO2, and 0.39±0.08 (σ) for CH3O2. This work suggests that the previously measured radical yield by indirect method for the ozonolysis of propene may be underes-timated. Radical production in the ozonolysis of propene in air was monitored directly by a peroxy radical chemical amplification (PERCA) instrument at room temperature (298 ± 2 K) and atmospheric pressure (1 × 10 5 Pa). The ozonolysis reactions were conducted in a flow tube under pseudo-first-order conditions for ozone. The decay in ozone was calculated based on reaction time tr and effective rate constant keff (keff = k1 [C3H6] 0)) for the ozone-propene reaction. A box model was used to simulate radical production in the ozone-propene reaction. The model-derived radical yields agree well with those from experiment within the uncertainty of the two methods. , the yields for OH, HO2 and CH3O2 were obtained from the measured total radical yields and radical partitioning derived from the box model. The yield is 0.39 ± 0.08 (σ) for OH, 0.19 ± 0.04 (σ) for HO2, and 0.39 ± 0.08 (σ) for CH3O2. This work s uggests that the previously measured radical yield by indirect method for the ozonolysis of propene may be underes-timated.