Abstract: The influence of the penetration depth of pile end into the bearing stratum on the ultimate bearing capcity of single pile is discussed and the tentative values of the critical depth and critical thickness of the bearing stratum are suggested too. All these are besed on the loading test results of four piles, two ending in silty sand （eleven-stage Loading tests） and two in stiff clay （twelve-stage Loading tests）, each stage being with different penetration depth into the bearing stratum. The ultimate bearing capacity of the piles, their friction and point bearing, have been estimated with static cone penetration resistance.