This paper investigates the bubble behavior of different heating methods of narrow rectangular channels. Comparisons were made on the onset of nucleate boiling (ONB) point and bubble behavior with various flow patterns. Results reveal that the wall superheat of two-side heating is lower than that in one-side heating at the same heat flux. This difference becomes obvious at a higher heat flux. Under the same subcooling condition, the required heat flux to start nucleate boiling for the one-side heating channel is higher than that for the two-side one. With similar bubbles behaviors, the cross-sectional subcooling tends to be higher for the two-side heating method.