Abstract: A fuel injection system of an outboard motor is provided with a vapor separator as a fuel tank disposed in the outboard motor and the system includes a fuel return pipe connected to the vapor separator, a fuel supply pipe connected to the vapor separator for supplying fuel to the fuel injector and a vent means connected to the vapor separator. The vent means is connected to a port formed to the throttle body through a connection hose on an upstream side of the throttle valve. The port formed to the throttle body is opened at a portion at which an air flows in the throttle body with highest flowing speed and upstream and downstream sides of the throttle valve are communicated through a by-pass passage which is provided with a port on the upstream side of the throttle valve opened near the port formed to the throttle body.