This thesis prepare in two major part which both are about constrained system. In the first part we analyze Hamiltonian structure, constraint structure, gauge transformation and the number of degrees of freedom of ”chern simons-like” gravity theories in 3 dimension. Since 3D gravity models in differential forms formulation have a brief and useful presentation we analyze these models in form formulation. The specific properties of 3 dimension also help us to accelerate the calculations. In this thesis we pay attention to the some unknown aspect of constraint structure of 3D gravity model which affect result. In 3D topological massive gravity (TMG) and New Massive gravity (NMG) are two major models. We analyze Hamiltonian structure of these models and obtain degrees of freedom for them which confirm pervious result and furthemore we obtain generator of diffemorphism and Local lorentz transformation (LLT) which known as symmetries of system. Beside these two massive gravity theory we nanlyse Hamiltonian structure of zwei-dibrain gravity (ZDG) which known as a bigravity theory. There is a conflict about ZDG is a ghost free model or not. Creator of models believe that model has 2 degrees of freedom which is good by some other people believe that model has 3 degrees of freedom and ghost mode is active. Our Hamiltonian analysis confirm that ZDG is ghost free model. In the second part of this thesis we proffer a method which help us to suggest Lagrangian for a system with known symmetries and apply this method for obtaining Lagrangian suggested for dir=rtl align=right which by choosing specific value reduce to known Lagrangian for dir=rtl
