Mechanism of transition delay by porous surface in hypersonic boundary layers is investigated. Linear stability theory (LST) is adopted to analyze characteristics of instability and comparisons with direct numerical simulation (DNS) are made. Under conditions Ma=6.0,Re=2.0×10⁴ (length scaled by displacement thickness of boundary layer),typical flow characteristics are obtained for planar boundary layers with and without distributed pores. Disturbing growth rate of smooth and porous boundary layers are computed by LST. N-factor,which is regarded as a symbol of accumulated growing of disturbance,is calculated. It shows that pores prohibit growth of Mack's mode and therefore delay transition of hypersonic boundary layer. Sequence-distributed pores stabilize boundary-layer flow more effectively than interlace-distributed pores.