The variation of the electrical interaction energy between two spherical particles as a function of particle size is investigated, We show that, for a fixed electrolyte concentration, the larger the particle, the greater the primary maximum of the total interaction energy; i.e., larger particles are less probable to coagulate than smaller particles, On the other hand, the smaller the particle, the higher the critical coagulation concentration (CCC). If the radius of a particle is larger than 1 mu m, CCC is independent of particle size, For larger particles, DLVO theory will overestimate CCC, and the reverse is true for smaller particles, Also, the higher the surface potential, the more serious the deviation, If the radius of a particle is sufficiently large, DLVO theory predicts that, kappa h(m) = 1, kappa and h(m) being, respectively, the reciprocal Debye length and the closest surface-to-surface distance between two particles, On the other hand, we show that kappa h(m) congruent to 8/7. The deviation of the DLVO theory arises from applying the superposition principle in the estimation of the electrical potential.