Abstract: Linear stability theory and e^N method are used to investigate the stability and transition of high-speed planar boundary layers subject to different wall temperatures and total temperatures. The present study extends the conventional understanding of the effects of temperature, which is helpful for transition correlation between different conditions. The temperature influences the boundary-layer transition in two aspects. Firstly, varying temperature results in different modal branching between fast and slow modes. This further affects the evolution path of infinitesimal disturbance. Secondly, temperature determines the fundamental instability of boundary layer, which leads to the variation of transition onset. It is revealed that the branching type changes under different wall and total temperatures. And the increase of total temperature is shown to stabilize the boundary layer. Besides, it is also discovered that the growth of disturbance varies exponentially with dimensionless wall temperature when the wall temperature is less than 1000 K. The transitional Reynolds number is formulated as a function of dimensionless wall temperature and N factor with the help of e^N method. Based on the formula, it is possible to evaluate the transition of boundary layer subject to specific initial disturbance and wall temperature. It also provides a method to correlate the transition results between different wind tunnels or between ground tests and flight tests.