Abstract: A novel technique based on optical theory and electromagnetic theory is presented to improve the gain of the dielectric plate len. The mechanism of gain enhancement via a dielectric plate len is analyzed with the presented method. The results show that the method is more efficient and stable than the traditional ones. A parametric study on doubledeck microstrip antenna covered by a dielectric plate len is performed to validate the method. Both the theory analysis and the numerical simulation indicate that dielectric plate lens with appropriate electromagnetic parameters can not only improve the gain of the microstrip antenna significantly, but also greatly reduce the size of the antenna array.