The traditional view is that temperature is the intensity of thermal motion of a great number of molecules. However, it can not interpret the existence of the constant geothermal gradient reasonably. This paper advances that temperature is composed of the thermal motion of a great number of molecules and microparticles. The density of microparticles in the earth is closely related with gravitational field, and the temperature distribution in the earth is the function of gravitational field. The density of microparticles in the earth is larger than that on the surface;the temperature in the earth is higher than that on the surface. This paper not only improves the thermal vibration theory, but also finds out the reason that geothermal gradient keeps constant.
Keywords: temperature, measure, geothermal gradient, thermal motion of molecules