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Zero moment point has been usually employed for balance keeping and walking analysis of active biped robots with bent knees, while balance and walking stability analysis of passive robots has been done using cyclic analysis approaches. While the problem in general is common, the two approaches have been separated in the literature for two different classes of passivity-based and ZMP-based robots. In this paper we have made a ZMP analysis for a passivity-based biped robot. The robot has been simulated using MSC Adams and Matlab/Simulink and a stable walking is achieved for a defined walking policy which is based on passivity-based walking with hips and ankles actuated and knee joints considered passive. The trajectory of ZMP is extracted and discussed during a stable walking. The role of Achill's spring on ZMP is discussed and the overall profile of ZMP trajectory and its changes at specific crucial events, like knee strike and heel strike, is analyzed.