Evolutions of collection efficiency and pressure drop of an air filter with dust load vary with the filter structure, such as mean fiber size, fiber size distribution, packing density, as well as the captured particle size and filtration velocity. In the present work, time change in collection performance of medium performance air filters composing of binary fibers is studied in order to clarify the effect of fiber size distribution on the collection performance. As a result, it was found that, during the period of depth filtration, the time change in collection performance of binary filters can be predicted by assigning unique fractional collection efficiency raising factors, overall collection efficiency raising factors and pressure drop raising factors of individual fibers, suggesting that there are no interactions between fibers with different sizes. This study confirms that mixing fibers with various diameters is one of the effective means to adjust the time change in collection efficiency as well as initial collection performance.