An experimental investigation on the thermohydraulic parameters of the Kartini research reactor by varying the primary pump flow has been conducted to study the behavior of fuel and coolant temperatures and heat transfer. The variation of the primary pump flow was physically simulated by gradually reducing the pump rotation while keeping the reactor power at maximum. An experimental tool equipped with seven thermocouple sensors and a Data Acquisition System was inserted into the hottest channel of the reactor core to obtain accurate data of the coolant temperature in the reactor core. This paper presents detailed results and analysis of the investigation. The main result is a unique pattern arising from the fuel and coolant temperature data, which allows us to identify three stages: flow-dominated, chaotic or transition, and natural circulation stages. Based on the difference between the temperature prediction and experimental results, we develop a novel correction factor to quantify the influence of the variation of the primary pump flow on fuel temperature prediction.