A pump of an electronically controlled brake system. The pump includes a bore formed in a modulator block and connected with a suction port and a discharge port, a piston installed in the bore in a reciprocating movable manner and defining an inlet path therein, an inlet valve to open or close an exit-side end of the inlet path according to a position of the piston, and an outlet valve provided at one side of the bore, operations of the inlet and outlet valves being contrary to each other. The outlet valve includes a valve seat, a sleeve to surround the valve seat, an outlet ball interposed between the sleeve and the valve seat and used to open or close a path defined in the valve seat, and a valve cap provided at one side of the sleeve to isolate an interior from an exterior of the bore. One or more of the valve sheet, sleeve, and valve cap are formed by press-forming, to achieve improved assembly efficiency and reduced manufacturing costs.