Biological ferroelectric materials have great potential in biosensing, disease di-agnosis and treatment. And the glycine crystal is the simplest bio-ferroelectrics. We have investigated the polarizations of two phases of glycine crystals, namely the β- and γ-phases. Using density functional theory, we find the Polarizations of those two crystals are comparable to those of conventional inorganic ferroelectrics. And using further systematic molecular dynamics simulations with polarized crystal charges, we predict the Curie temperature of γ-glycine to be 630 K, with a required coercive field to switch crystalline polarization states of 1 V·nm-1, consistent with experimental ev-idence. This sheds light on the microscopic mechanism of electric dipole ordering in biomaterials, helping in the materials design of novel bio-ferroelectrics.