The behavior of the flow field in a rectangular micro-channel was numerically investigated using shear stress transport model. The γ-Reθt transition model, laminar model and shear stress transport model are used to comparing the flow transition in the micro-channel. Results indicate that the laminar model and the γ-Reθt transition model produce similar pressure drops when the mass flow rate is smaller. When the mass flow rate increases, values predicted by the γ-Reθt transition model matches best with the experimental data. The laminar model and the SST model are incapable of predicting transition, while the γ-Reθt transition model forecasts the critical Reynolds number well and the predicted values match well with the experimental data. In the present study, the transition is accurately simulated and the flow mechanism is revealed. For microchannels with L/D≥100, the transition from laminar to turbulent regime occurs for a Reynolds number in the range 2000-2500, and the length-to-diameter ratio has no significant effect on the critical Reynolds number.