The relationship between the spontaneous polarization strength and effective dielectric constant of BaTiO3 and its grain size has been calculated based on landau ginzburg devonshire(LGD) phenomenological theory in this paper.Lich,MGT,EMT and J S theoretical equations were used to simulate the effective dielectric constants of BaTiO3/Epoxy composite in the range of 1 nm to 1000nm.The results show that the single component specimen of BaTiO3 with grain size of 300 nm has a high effective dielectric constant at room temperature,and J S equation could be well fitted to the experimental data at low volume fraction(10%~40%) when it was composited with Epoxy.