OH-initiated photooxidation reaction of isoprene was performed in the home-made smog chamber. Isoprene and important gas phase products (methacrolein and methyl vinyl ketone (m/z=70) were monitored real time by proton-transfer-reaction mass spectrometer (PTR-MS), and formation and growth of secondary organic aerosol (SOA) were measured by scanning mobility particles sizer (SMPS). Photoionization mass spectroscopy of important gas phase products from this photooxidation of isoprene was obtained using synchrotron radiation photoionization mass spectrometry (SRPIMS). It is shown that with increase of reaction time, isoprene concentration reduced gradually, and SOA began to form and grow. When isoprene was depleted nearly, the mass concentration of SOA was still kept to grow, which indicated that heterogeneous reaction of the main gas phase products occurred on the surface of particles should be responsible for the growth of SOA.