Improved design of optical directional full couplers (ODFCs) based on new function modified by hamming function is investigated. The modification-hamming function weighting (M-HFW) of new function is described. The validity of approximation theory of optical directional full couplers becomes questionable when the coupling length is not enough long so the beam propagation method (BMP) is used. We propose a new design and numerically show that some optical properties of the optimized one are better than the optimized coupler with hamming function weighting (HFW). The objective of the work includes in the new design of optical directional full couplers can obtain respectively low noise, large frequency and short couplers. When the crosstalk is demanded to be smaller than -40 dB at testing wavelength 1.57 μm, the coupler length for hamming function weighting for new weighting is 5.8 mm and the one for modification-hamming function weighting is only 4mm. When the length of coupler is 12 mm and the crosstalk is smaller than -40 dB, the corresponding wavelength ranges of convention and modified cases are in the 1.38-1.67 μm and 1.3 -1.7 μm, respectively. It is surely that the proposed modification-hamming function weighting is more effective than convention weighting for optical directional full couplers.