Equal channel angular pressing (ECAP) was performed on the extruded commercial pure magnesium at 250 degrees C for 4 passes. Heat treatments were carried out to modify the microstructures. The cyclic plastic deformation behavior of pure Mg with different grain sizes in microstrain region was studied by tensile loading and unloading experiments. The microplastic deformation process of pure Mg can be divided into two stages. In the first stage, pronounced plastic deformation associated with dislocation motion on basal plane is initiated at several MPa. The materials are softened and characterized by low friction stresses and hardening exponents. The microplastic deformation enters into region II above the strain of about 8 x 10(-4). Annihilation and tangle of dislocations lead to the increase of hardening exponents and friction stresses. Pure Mg shows a very pronounced anelastic behavior during cyclic microplastic deformation, which results in a rapid increase of modulus defect, effectively decreasing the elastic modulus by up to 60 %. Grain size has a marked effect on microplastic deformation behavior of pure Mg. With increasing the grain size, the specimen shows a more pronounced microstrain and anelastic behavior.