To design a high-performance air filter with a longer service life, we need to predict how the morphology of particle accumulates on a constituent fiber changes and affects the collection efficiency and pressure drop of the filter under dust-loaded condition. This paper describes a practical three-dimensional simulation method for predicting the agglomerative deposition process of fine aerosol particles on an electret fiber. The simulated results are shown to agree quite well with the experimental observations for both uncharged and charged particles. For the former, only the gradient force, and for the latter, the coulombic force needs to be considered as long as an oncoming particle has not come in close proximity to any previously deposited particles. In contrast, once the oncoming particle enters a region of close proximity to a deposited particle at the tip of a dendrite or chain-like agglomerate, it suffices to consider only the high-gradient or particle string formative force in the present stochastic model.