Abstract: The rotation-release method is for determining the number of independent linear displacement unknowns in the structural mechanics. Yet due to the incomplete consideration of the constraints in the classic method, the application of the method is limited in many aspects. To overcome the limitation of the method, a modified rotation-release method is proposed, with consideration of all kinds of constraints and the effect of the rigid bars, which reflects more directly the “rotation releasing” and the assumption for the straight bending bar from the principle level. Examples show that the modified method can be accurately applied to solve very complex problems, and if the involved mass is properly considered, the modified method can also be used to determine the number of vibration degrees of freedom under a complicated situation.