Experiments were carried out to investigate the critical heat flux characteristics during flow boiling of deionized water in a micro channel with micro pin fins in the shape of circular, diamond, and ellipse, respectively. In the experiments, the mass flux ranges from 96 to 224 kg·m^-2·s^-1, the inlet subcooling ranges from 20 to 50 ℃, and the effective heat flux ranges from 10 to 240 W·cm^-2. CHF is caused by dryout at the wall near the outlet of the microchannels, which in turn is attributed to the flow reversal upstream of the microchannels. The occurrence of CHF is marked by an abrupt increase in wall temperature near the outlet and an abrupt decrease in pressure drop across the microchannels. In addition, it is found that the experimental parameters such as mass flux, inlet subcooling, and the shape of micro fins also have a great influence on CHF. The experimental results show that the critical heat flux with micro channel of micro pin fins is higher than that of a smooth channel and the existence of micro fin greatly reduces flow reversal and subsequent instabilities in flow boiling. The CHF of the elliptical micro pin fins is the largest, and the circular micro pin fins is the smallest. The CHF increases with the increase of mass flux and inlet subcooling, but decreases with the increase of outlet quality. The experimental data are compared with the correlation proposed by Kosar et al. The results show that the experimental data are in good agreement with the correlation.