In polymerization involving a rapid exothermic reaction, it is necessary for the generated heat in this operation to be removed from the reactor by a cooling coil or jacket to control the reaction temperature. But, fluid in the reactor is gets stagnant as the reaction proceeds, because the viscosity is increasing due to monomer conversion, therefore the reactor often has induced uneven temperature distribution. In this work, radical addition polymerization was carried out in a reactor using several types of impeller-paddle, anchor, helical screw and helical ribbon, Under these conditions, local temperature distribution was measured in detail using our prototype real-time and multi-point temperature measuring instrument which is able to measure simultaneously changing temperature at local positions via many thermocouples. As a result of these experiments the condition of changing local temperature and the obtained polymers were found to be related to the type of impellers. We found the high temperature areas in the reactor produced polymers composed of undesirably short chain length molecules. As the cooling condition of the reactor was found by measuring local temperature, we could also find a suitable position for the control sensor of temperature for lowering local higher temperature in the polymerization under the set value.