Poly (methyl methacrylate) (PMMA) microspheres were synthesized by suspension polymerization. Two types of thiol compounds, namely normal dodecyl mercaptan (DDM) and tert-dodecyl mercaptan (TDM) were used as chain transfer agents (CTA) to investigate their effects on controlling the molecular weights and their distribution, morphology and particle size. Cold gel permeation chromatography (GPC) analysis was used to determine molecular weight distribution (MWD). The morphology of microparticles was analyzed by scanning electron microscopy (SEM) size and their distribution were measured by dynamic laser scattering (DLS). Synthesized microspheres accompanied by 0.48 phr of DDM and TDM had a mean average volumetric size of 433 and 304 µm, respectively. The results revealed a Gaussian molecular weight distribution for synthesis particles with TDM, whereas particles produced in the presence of DDM showed bimodal MWD, so that one peak less than 100,000 and the other peak more than 1000000. On the other hand, SEM images revealed smooth and transparent surface for those with TDM samples. On the contrary, rough surfaces were observed for samples with DDM. DSC analysis revealed similar thermal behavior for all the samples with identical glass transition temperature around 122˚C. TGA results manifested a significant improvement in thermal stability of samples in the presence of CTA.