This paper reports the development and evaluation of a ultra-small and ultra-light wireless sensor node for real time monitoring of biological information of animals. Power consumption of the sensor node was decreased dramatically by improving efficiency of the MCU and irradiation power of the RFIC, which enables a few months life time when using SR416SW batteries. Various configurations of the antenna were investigated for the pursuit of small package size, while maintain an enough communication distance of a few meters. The implant experiments were carried out to investigate performances of the sensor node as well as practicability of the package. Moreover, the feasibility study of a wireless power transmission module as the power source was performed and discussed herein.