The effect of adding nanofibers to a non-woven microfiber filter media is determined theoretically. The filter performance is characterized by a Relative Quality Factor and the amount of nanofiber added to the filter is measured as the ratio of surface area of nanofibers per surface area of microfibers. Calculations show the Relative Quality Factor passes rises rapidly with addition fo small amounts of nanofiber, reaches a maximum, and gradually decreases as more nanofibers are added to the medium. This shows that the amount of nanofibers mixed into the microfiber can be selected to optimize filter performance.