Abstract: According to the suggestion of Sader et al.,who replaced the reduced surface potential Y=eψ/KTby the effective reduced surface potential Y(h)=4e^kh/2·tanh^-1[e^kh/2·tanh (Y/4)](where ψis the surface potential of particles,k is Boltzmann's constant,Tis absolute temperature,h is the distance of closet approach between the particles,e is the proton charge,k is the Debye screening parameter),we return to the double layer interaction at moderate potential under the constant surface potential,and using improved Derjaguin method we derive formulae for the interaction energy between the two parallel plates,valid up to the moderate to high potential regime.Agreement with the exact numerical values of the interaction of idential or dissimilar plates is considerably improved for the moderate surface potential.

Key words: Repulsion energy, Plane colloid particles, Electrical double layers, DLVO theory