Compared with conventional channels, narrow and micro channels have significant characteristic of heat transfer enhancement. With smooth internal surface, such channels can efficiently avoid encrustation at the washing effect of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, much work was focused upon flow patterns, heat transfer and pressure drop. Almost everyone thought the heat transfer enhancement mechanism of narrow and micro channels to be bubbles’ deformation and disturbance, which is insufficient to explain the heat transfer enhancement. In present work, an innovative model of quasi-one-dimensional vapor liquid two-phase concurrent separated flow was proposed for boiling heat transfer in vertical narrow rectangular space. Numerical results such as boiling heat transfer coefficient and liquid film thickness were obtained. Comparison of model results with reported experimental correlation indicates that the proposed model can predict heat transfer in narrow channels correctly, with the relative deviation less than 14%. Numerical simulating result confirms that heat conduction through liquid film is the predominant mechanism of boiling heat transfer in vapor liquid separated flow region in a vertical narrow rectangular space.