Coir fiber is derived from the coconut shells and considered to be an economical and natural material. In order to further extend its application fields, the geometry and microstructure of the coir fiber were thoroughly investigated in this research. The single fiber length measurement was carried out, and it was revealed that the length of the coir fiber was between 8 and 337 mm. Length distribution of the coir fiber was simulated by using the computer technology, and was found to be in coincidence with that of most natural fibers. The grouping measurement analysis was used to study the length-weight distribution and length-linear density distribution of the fiber. It was found that the average linear density was 27.89 rex and the linear density was between 18. 265 and 70. 442 tex. The length-weight distribution of the coir fiber showed a Poisson pattern, and the weight of fibers with the length between 50 and 230 mm accounted for 85. 28% of the total fibers measured. In this research, scanning electron microscope (SEM) was applied to observe the morphological changes of coir fiber before and after alkali treatment; and the result shows that alkali treatment leads to the removal of lignin and pectin from both the cuticle and the inside walls, which is beneficial for the interfacial adhesion with polymer matrix in composite fabrication.