The critical minimum size of stable precipitate in the vicinity of edge of miscibility gap is experimentally determined for the Ni3Si Precipitate particle in Ni-Si, Ni3Al in Ni-Al, Cu4Ti in Cu-Ti and Co in Cu-Co binary alloy systems by utilizing the macroscopic composition gradient method recently proposed. The results obtained are as follows: The critical nucleus size shows a rapid increase to several tens of mu in a very narrow composition region less than 0.3at% from the phase boundary. Such a big critical size of nucleus is statistically rationalized by the conventional nucleation theories.