Bubble behavior in horizontal ultra-shallow microchannels displays distinct features in flow boiling heat transfer process, which is embodied as the fast elongation and deformation. It expands the range of the microlayer region and causes effective disturbances to the main flow, and therefore contributes to the enhancement in heat transfer. However, too fast deformation of the confined bubbles also results in rapid reducing of the liquid film thickness, causing the early appearance and short span of the annular flow. The enhancement in the heat transfer capability is then limited or even weakened. These distinct features lead to unsuitable predictions of classic correlations for the experimental data. The average error of the classic correlations exceeds 50% and the largest deviation reaches 125%. On the ground of the visualization experimental results, a theoretical model based on Chen's model is proposed to calculate the flow boiling beat transfer coefficient in horizontal ultra-shallow microchannels. The presented model can be applied for both the elongation bubble flow and the annular flow as the heat transfer mechanism for these two flow patterns are specially considered respectively. Verified by the measured data, the average error of the model successfully drops to +/- 15%, from +/- 52.4% of the Chen's model whose prediction fitted experimental data the best among the classic correlations. (C) 2016 Elsevier Ltd. All rights reserved.