The fallen part hit the fifth floor of a building in the Lumpini Park Riverside project. Four male workers were killed on that floor and another five injured.
The accident happened at about 12:15pm on Soi 45 off Rama 3 road.
A worker said he heard a loud noise followed by a large volume of dust appearing. He looked up and saw that the top part of the crane had broken off.
Officials have yet to investigate the cause of the accident.