We consider the two-point, two-time (space-time) correlation of passive scalar R ( r, x) in the Kraichnan model under the assumption of homogeneity and isotropy. Using the fine-gird PDF method, we find thatR ( r, x) satisfies a diffusion equation with constant diffusion coefficient determined by velocity variance and molecular diffusion. Its solution can be expressed in terms of the two-point, one time correlation of passive scalar, i.e., R ( r, 0) . Moreover, the decorrelation of R ( k, x), which is the Fourier transform ofR ( r, x), is determined by R ( k, 0) and a diffusion kernal.