The effect of environmental factors on Secondary Organic Aerosol (SOA) formed from hydroxyl radical (OH·) initiated photooxidation of isoprene were investigated by using a home-made smog chamber. The size distribution of SOA particles from the above reaction were measured using Aerodynamic Particle Sizer spectrometer (APS). The effect of radiation time, reactant concentration, and CH3ONO concentration on SOA formation from isoprene were evaluated by performing photooxidation experiments under varying conditions. Experimental results showed that aerodynamic diameter for SOA detected in isoprene photo-oxidation experiments was predominantly in 0.4 mm~1.4 mm, which were in the form of fine particles with diameters less than 2.5 μm. These fine particulate matters are more easily deposit in the lung of the human being, and do great harm to the health. This paper also highlighted that both the number and mass concentration of SOA particles were increasing with the prolonging of the radiation time, the increasing of light intensity, and the increasing of the reactant concentrations. The results could provide useful information to infer possible emission sources of atmospheric particles in future field measurement.