The relation between the film thickness and the growth conditions in the liquid phase epitaxy （LPE） process is discussed. A convolution approach for the thickness is developed on the assumption that the growth rate is determined by the solute diffusion process. Using this convolution expression， the relations between thickness， growth time and cooling rate can be obtained for various LPE techniques. Moreover， the convolution algorithm can also be used to deal with some complex growth conditions， such as nonuniform cooling rate， nonlinearity of the liquidus curve and the finite growth solution.