ZMP(Zero Moment Point) and(Center of Pressure) CoP are important criteria for the stability of the biped locomotion. In this paper, the relation between ZMP and CoP is studied. Based on the posture adjustment requirement of THBIP-I and the installation position of the 6-axis force-moment sensors, the formulae for computing CoP of a humanoid biped robot are deduced, and a CoP measurement system based on the 6-axis force-moment sensors is presented. The accuracy of this system is experimentally confirmed, and CoP is measured throughout the whole walking phase. The results of measured CoP are discussed.