In this paper, an analysis on force data during needle insertion for biopsy is conducted. The analysis proposes a novel way of assessing the organ stiffness. During the biopsy operation, the force data is recorded by constructing reaction force observer (RFOB) without utilizing force sensors. The recorded force data is then treated in the force modeling equation of needle insertion movements. Finally, the quantitative stiffness assessment method is presented. This paper utilizes the force modeling into three phases, which are contact, puncture, and pierce phases. The analysis to model the force data for each tissue inside the content is conducted. The analysis is discussed to evaluate the stiffness of three different environments which all imitate the liver.