Deterioration in the filter removal efficiency of submicron particles ( λ/ λo) under unfavorable surface conditions is affected by the number of deposited particles per filter grain. In the case of above micron particles, the deterioration of filter removal efficiency has been mainly due to the blocking effect of deposited particles and not by the number of deposited particles. Deposition of large number of submicron particles changed the surface characteristics of collectors (filter grain associated with deposited particles) and enhanced unfavorable surface conditions. Filtration experiments were conducted with monodispersed suspensions of known sizes of submicron latex particles at different ionic strengths, using glass beads as filter grains. The filtration performance was predicted by using a mathematical model, assuming a linear relationship between λ/ λo and σ (i.e. λ/ λo=1.k σ). For both particles, k was found to decrease and λo was found to increase with the increase in the ionic strength. A comparison was made of the importance of blocking effect for the filtration of submicron particles.