A motorcycle body frame includes a main frame including a head pipe for supporting a front suspension at the front end thereof, a single leading frame portion extending rearwardly from the head pipe, and a rear frame portion for supporting a rear wheel suspension. The rear frame portion extends substantially downwardly from a rear part of the leading frame portion. The main frame, cast from an aluminum alloy, includes two component parts arranged front-to-rear. For example, a front frame portion is provided, and a rear frame portion is fixed to the rear side of the front frame portion. The leading frame portion has a small section portion, where the main frame is smallest in sectional area, disposed substantially centrally in the longitudinal direction thereof. The two component parts of the frame are joined at a coupling portion of the main frame situated in back of the small section portion.