Abstract: Alkene ozonolysis products are precursors of secondary organic aerosol (SOA) formation. In the present study, the formation of transient species in the ozonolysis of cyclopentene was investigated using the self-established system of matrix isolation in the condensed phase at temperatures ranging from 6 K to 115 K. In the experiment, the reactive intermediates were monitored using MI-FTIR during heating-up, and band assignment and structure identification were confirmed, combined with theoretical calculations and reported literature. The experimental results showed that POZ, SOZ and CI intermediates were successfully detected. The characteristic peaks of POZ at 705 and 946 cm-1 were assigned to O—O—O asymmetric stretching and ring deformation, respectively. While the characteristic peak of SOZ at 939 cm-1 was assigned to C—O stretching, those of CI, at 1733 and 853 cm-1, were assigned to CO stretching and O—O stretching, respectively. The results effectively supported the Criegee mechanism of the cyclopentene ozonolysis. The computed vibrational frequencies with four methods of B3LYP/6-311++G(d,2p), B3LYP/6-311++G(3df,3pd), MP2/aug-cc-pvdz and PW91PW91/6-311++G(3df,3pd) were compared with the experimental values. The measured value of POZ absorption peak was at 705 cm-1, and the results of four computational methods were at 725,8, 703 and 680 cm-1 respectively. The measured value of SOZ absorption peak was at 1030 cm-1, and the results of four computational methods were at 1017,8, 1025 and 993 cm-1. Also, the measured value of CI absorption peak was at 1733 cm-1, and the results of four computational methods were at 1796,3, 1732 and 1745 cm-1. The ab initio MP2/aug-cc-pvdz method showed very good performance in predicting characteristic vibrations and agreed well with the experimental data.