The definition, measuring methods and principle of size-of-source effect (SSE), and also the measuring principle of radiation thermometer are introduced in this paper. The SSE results with direct and indirect method of RT9032 and LP4 are shown in this paper. The data of SSE got by indirect method is bigger than that of direct method in our measurement system. There are several aspects contributed to the difference of SSE, such as the reflections from the inner cone and edge of the aperture into the black spot, the radiance of the sphere reduced by black spot, and other aspects including the reflections between the front element of the thermometer and the black spot, optical misalignment radiation thermometers and so on. If SSE is measured by direct method employed the integrating sphere, the monitor is necessary to revise the drift of the radiation.