Acute lymphoid leukemia is a common type of blood cancer and chemotherapy is the initial treatment of choice. Quantifying the effectiveness of a chemotherapeutic drug at the cellular level plays an important role in the process of the treatment. In this study, an optical tweezer was employed to characterize the mechanical properties of Jurkat cells exposed to artesunate (ART) as a chemotherapy. A mathematical model was developed to describe the mechanical characteristics of the cell membrane and its features. By comparing the modeling results against experimental results from the optical tweezer, the elastic modulus of the Jurkat cells before and after ART treatment was calculated. The results demonstrate an increase in the cell stiffness after treatment. Therefore, the elastic modulus of a cell membrane may be a useful biomarker to quantify the effectiveness of a chemotherapeutic agent.