An experiment study is conducted to investigate the effect of transverse power distribution on the Onset of Nucleate Boiling (ONB) through a one-side heated narrow rectangular channel. Two test section are used to perform the experiment; uniform and non-uniform heated suction. The demineralized water is flowing in upward direction through the coolant channel with a thickness of 2.35 mm, a width of 54 mm, and a length of 300 mm. The experiment is carried out under different thermal power (0.5 kW – 6.5 kW) for the both test section. As well as, a wide variety of inlet subcooling and flow velocity are used as; 65−35 °C and 0.1–1.0 m/s, respectively. The wall temperature distribution of the heated plate is measured by 10 TCs for the uniformly heated test section, and 20 TC for the non-uniformly heated section. On the other hand, the ONB location is visualized via high speed camera, in which the ONB occurs near the edges for the non-uniformly heated section and occurs at the center of the heated surface for the uniformly power distribution. The results of the ONB heat flux and temperature in the non-uniformly heated section are compared against the one in the uniformly heated power. The results show the variety of the ONB location, ONB heat flux with the different power distribution. With the increase of the power, the ONB is shifted toward the inlet. On the other hand, the ONB for the non-uniform power distribution occurs near the edges at power lower than that the one in the uniformly power distribution. Also, the results are compared against the available correlations, such as Bergles and Rohsenow (1965), Jens and Lottes (1951), and Thom et al. (1965), as well as other experimental results done by several research institutes.