The purpose of the work was to estimate an influence of gas-assisted injection conditions (the temperature of plastic material, switch time-delay time, gas injection time) of shape, position and dimension of gas channel, and structure of injection molded parts. The change in the value of the dynamic Young modulus and the mechanical loss tangent in function of temperature and oscillation frequency by the dynamic mechanical thermal analysis (DMTA) method was determined. It was found that injection molding parameters: injection molding temperature, switching time and gas injection time influenced significantly mass, wall thickness, and thermomechanical properties of parts. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers