We observed that Talbot pattern in Gaussian beam passing through aholographic grating may carry information of the beam pot size. A scheme for measuring Gaussian beam diameter based on Talbot effect is presented in this paper. By knowing the space of frings of Talbot pattern, the beam spot size projecting on the holographic grating can be obtained. This characteristic reduces the requirements to location of grating or light source and the stability of electrical circuits and light source. In this paper we describe the phenomena as a convergent Gaussian beam passing through holographic grating, and analyze the possbility of the application of Talbot effect to measuring beam diameter. Theoretical analysis, computer simulation and experiments results agree well with each other.