Thermoforming is one of the new methods for forming of polymer sheets. Free thermoforming is one of the thermoforming methods in which shaping is done with air pressure or vacuum without the plug mold. In this paper, free thermoforming of Poly Methyl Methacrylate (PMMA) has been investigated by experimental tests and finite element simulation. The main purpose of this article is the identification of the real behavior of PMMA during free thermoforming to achieve maximum workable air pressure with respect to initial thickness. For this, at first, tensile and relaxation tests have been done in working temperature (160◦C). Then the process was simulated by Abaqus software with considering four types of the material property: three hyperelastic models (Ogden, Mooney-Rivlin, and Marlow) and a hyperviscoelastic model. After that, experimental tests were done, and the samples final shape were compared with simulation results. Accordingly, the simulation results obtained based on the Marlow hyperelastic model showed the best agreement with the experiments compared to others. After that, maximum workable air pressure versus plate initial thickness and minimum thickness of the deformed plate were achieved by finite element simulation.