Abstract: A filter assembly has a housing and a filter. The housing has a base, a cover with a outlet, an outer filter screen being tubular, an inner filter screen being tubular and a metal net being tubular. Lower edges and upper edges of the outer filter screen, the inner filter screen and metal net are respectively fastened on the base and the cover. The inner filter screen is surrounded by the outer filter screen. The metal net is surrounded by the inner filter screen. The filter is mounted between the outer filter screen and the inner filter screen and fitters impurities even with small dimension. Thus, the filter assembly will remove the impurities clearly to prevent a cylinder and a piston in an internal combustion engine from being rubbed.