Numerical investigation was conducted on a typical two-dimensional hypersonic inlet to study the influence of unforced boundary layer transition affected by compression ramp geometric parameters on the inlet performance. The numerical results show that the transition onset location on the compression ramp can be delayed by filleting the ramp intersection, and also the inlet's performance obviously improves when the transition onset location is delayed. Compared with full turbulent situation, when the boundary layer transition occurs, the unstart of the inlet is significantly mitigated, the heat transfer rate on the compression ramp decreases, both the total pressure recovery coefficient and mass flow rate increase at both design and off-design points. But the static pressure distribution along the ramp is fairly independent of the varieties of boundary layer.