The effects of addition of solvent on a polymerization reaction process were investigated taking into consideration temperature distribution, physical properties and conversion in the reactor. In the experiments, a mixture of styrene monomer, solvent and initiator for the reaction was fed into a vertically-standing tubular reactor equipped with, electrical furnaces to measure the polymer conversion at the reactor outlet. When the initiator was added, the decrease in the conversion caused by the addition of the solvent was smaller than the case in which no initiator was added. The measured dependences of the conversion on experimental conditions were explained well by numerical simulations for momentum, heat and mass transfer accompanied by the polymerization reaction, which took account both for the change in the physical properties with temperature and conversion, and for the heat production by the polymerization reaction. The simulation results showed that the addition of the solvent caused the distributions of axial velocity and viscosity to be smaller. The results also indicated that such a change in the flow field in the reactor led to a change in the distributions in residence time and conversion of styrene in the reactor, which suggested the importance of considering the transport phenomena in a reactor for evaluating the effects of solvent on a polymerization reaction process.