Order-disorder phase transitions for CH3NH3PbC13 are studied with density functional theory.Our calculations show that the disorder is manifested in two aspects in the cubic phase,namely,the disorder of orientation and rotation of organic groups.Organic groups of [CH3] and [NH3] in cubic crystals can easily rotate around its C3 axis.At the same time,[CH3NH3]+ organic groups can also orient to different spatial directions due to the weak interactions between organic group and inorganic frame.Our results show that its possible phase transition path starts from the deviation of organic groups from the crystal c-axis.Its structural transition changes from disordered cubic phase to hydrogen-only disordered tetragonal structure in the process of decreasing symmetry.The disordered high temperature cubic phase can be expressed as a statistical average of substructures we rebuilt.The electrostatic repulsive force between adjacent organic groups triggers out the formation of low temperature phase on cooling.