A mount frame includes a first frame unit having a first rear frame portion which is provided with an anchoring member and a gripped member. A second frame unit has a second front frame portion provided with a seat post, an anchored member which is disposed to be journalled on the anchoring member about a rotational axis. Thus, the second frame unit is rotatable relative to the axis to facilitate assembly of the first rear frame portion and the second front frame portion. A gripping member is disposed on the second front frame portion to confront the gripped member. A tightening unit is disposed to push the gripping member to abut tightly against the gripped member.