Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas-liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen-water flow in a microchannel with the size of 1500 x 720 mu m were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m(2)-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m(2). The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.