We present a study of full passive magnetic human body communication for temperature sensing application. To generate the magnetic field of the human body, specified inductive couplings are employed to model human arms. The system contains Transmitter, Tag, and Receiver. We focus on the design of Tag, which is no-battery. It works on the environment of the magnetic field of human body. To validate the effectiveness of the whole system, Tag is attached to the human body. Due to the linear relation between the frequency of ring oscillator and temperature, this Tag could be regarded as a temperature sensing application.