The status of functional performance (FP) is a key component in making a decision about termination or continuation of adjuvant chemotherapy. A questionnaire-based method known as eastern cooperative oncology group(ECOG) is often utilized to assess the FP of cancer patients during adjuvant chemotherapy. The ECOG is primarily a subjective method and consequently prone to error and inaccuracy. This paper proposed an objective FP assessment method using an array of 23 body-mounted inertial sensors collecting kinematic motion data during walking test (WT). A Gaussian mixture model (GMM) was subsequently applied to data to identify postural states representing the adjuvant chemotherapy effects. The method applied to a prototypical model simulating pre- and post-chemotherapy FP was developed in consultation with an oncologist. The method was subsequently validated on a breast cancer patient. The results were promising as they can clearly and quantitatively identify the effects of adjuvant chemotherapy on FP and can potentially assist clinicians to increase the treatment accuracy.