Abstract: A simplified heat transfer model for heating phase in rotational molding process was established upon the heating stage from the initial heating of mold to the initial melting of powders. The heat transfer coefficient between the inner mold wall and internal mixture was obtained from the unsteady heat conduction equation when the inner mold wall contacted the internal mixture. The differential equations for the model were solved with the aid of MATLAB software. This model was validated by comparing the experimental results with the calculated temperature of internal air. The effects of heating temperature inside the chamber, outer heat transfer coefficient and the thickness of plastic part on heating time and heating efficiency were investigated by use of this model. The results indicated that the heating efficiency increased with an extension of time, and it was higher for thicker plastic parts.