In this paper we are proposing a needle-type GMR sensor to estimate magnetic fluid weight density inside large tumors. The application is hyperthermia therapy, a form of cancer treatment. Hyperthermia therapy utilizes the magnetic losses due to magnetization of magnetic nanoparticles by external alternating current magnetic fields. These magnetic losses can be dissipated in the form of heat depending on the thermal conductivity and heat capacity of the surrounding medium. The overall effect is an increase in temperature of the surrounding. This principle is applied to heat and destroy tumors since they are more sensitive to heat than normal healthy cells. Generally all parameters except the magnetic fluid weight density are known in the specific heat equation which governs the heat given in hyperthermia therapy to destroy cancer cells. Hence, accurate estimation of magnetic fluid weight density is critical for successful treatment. This paper presents a methodology to estimate magnetic fluid weight density inside the body and experimental results, by a needle-type GMR sensor.