Research reactors should incorporate the measurement system for the core power to control and regulate the reactor power. The open-pool type research reactor modeled in this paper has three seperated and independent channels of the neutron detectors to measure the core power. To calibrate these detectors, the thermal power of Primary Cooling System (PCS) which cools down the heat generated in reactor core is used as claibration reference. The core thermal power can be estimated by the measured values of the mass flow rates, core inlet/outlet temperatures of coolant in the PCS. In general, the uncertainty of the core thermal power is required to be controlled below a certain value. To meet this requirement, the uncertainty of core thermal power should be evaluated based on the uncertainty of the measured parameters. In this paper, the calculation equation for the core thermal power is derived and the uncertainty evaluation is conducted with variation of the uncertainty of the measured parameters such as mass flow rates and temperatures. In this parametric study, the allowable uncertainties for measuring devices have been obtained to guarantee 5% of the core thermal power uncertainty.