A frame unit of a curtain wall has a mullion, a beam and a connecting device. The mullion has a mullion recess and a mullion channel. The beam is substantially the same as the mullion and has a beam recess and a beam channel. The connecting device has a first group and a second group. The first group has a plate, two chunks and two bolts. The plate is mounted in the mullion recess. The chunks are mounted in the mullion channel. The bolts are mounted through the plate and are respectively screwed into the chunks. The second group is the same as the first group and is similarly connected with the beam. Because one beam can be connected with and spreads forces to multiple mullions, a structure of a curtain wall can be enhanced.