Evaluation of thin liquid films due to its many applications in biophysics, physics and engineering, as well as interesting natural phenomenon has great importance. These contents could include common liquids such as water and oil or more complex materials such as polymer solutions or melts, and the mixture of different phases. Subjected films to the various mechanical and thermal factors show interesting phenomena such as wave propagation, wave steepening, fingering and spreading. From industrial point of view, this subject is applicable in the process of slivering, optical coating, lubricating and adhesive. In this thesis, thin liquid films have been studied considering Newtonian and non- Newtonian liquids. In order to model the rheology of non-Newtonian fluid, power-law model has been used. At first, assuming long-wave approximation, we simplified the momentum and continuity equations for thin liquid films. Then, considering appropriate boundary conditions, we solved system of equations in order to derive the evolution equation of thin liquid film for power-law non-Newtonian liquids. This evolution equation is a highly nonlinear equation which is derived in three- and two-dimensional forms. After that, considering the simplest case in which the film is supported from below by a solid surface and subjected to the influence of gravity and constant surface tension, evolution of thin films are investigated. In this section, the stability properties of a uniform film perturbing by a small periodic disturbance have studied. Then using a numerical method, we solved the evolution equation of thin film of power-law non-Newtonian liquids and the results are compared with Newtonian liquids for different Bond number. It is shown that the weakness of power-law model in low and high rate of strain affects the result in this case. Furthermore, the results show that the evolution speed strongly depends on the fluid’s rheology. Rayleigh-Taylor instability is another subject that is studied in this work. This interesting phenomena in which gravity force acts a source of instability in the liquid films’ motions, is investigated with solving the evolution equation numerically for different Bond number. In this section, behaviors of pseudoplastic, dilatant and Newtonian fluids have compered and the results show that the evolution of free surface for pseudoplastic fluids is different from Newtonian and dilatant fluids. Also, the effect of wavelength of initial disturbance on the evolution of thin liquid films is investigated. Key words Thin liquid film, Newtonian fluid, non-Newtonian fluid, Rayleigh-Taylor instability, Power-law model