Abstract: In the reactor engineering,narrow channels was used widely for its high power density,exceptional heat transfer and actual engineering requirements.The point of Onset of Nucleate Boiling（ONB） is the key point of boiling heat transfer in narrow channels.The point of ONB can directly influence the fol-lowing flow and heat transfer characteristics in the reactor.Due to the special structure and complexity flow,the point of ONB in narrow channels are effected by many factors,which characteristics are not understood completely yet.Using B＆R model,Su Shun-yu model,Pan Liang-ming model and Yang Rui-chang model,the heat flux of onset of nucleate boiling is compared and analyzed by taking water as the medium.And then the relationships of the heat flux with pressure,mass flow and wall temperature are obtained.Based on the differences of each model,the mechanisms for the main influence factors are suggested..