Aggregation of fly ash particles with size range of 0.023-9.314 μm in a uniform magnetic field was investigated for removing them. A binary collision-aggregation model evaluating particle aggregation coefficient was developed. Based on the model, particle removal efficiency was calculated by solving the General Dynamic Equation. The comparison was done between the calculated and experimental data. The modeling aggregation coefficient shows that the aggregation coefficient increases with particle size, and the bigger the size difference between two particles is, the more strongly the gravity difference promotes aggregation. For the mid-sized particles, their removal efficiencies are higher than those of the smaller and bigger ones. The effect of the magnetic flux density on total particle removal efficiency is similar to that on aggregation coefficient. Before particles are saturatedly magnetized, their total removal efficiency increases with an increase in the magnetic flux density. Higher removal efficiency can also be caused by prolonging the particle residence time in the magnetic field or increasing their mass concentration. The particle number median diameter decreases with an increase in the total removal efficiency. Calculation results are found to coincide essentially with the experimental data.