We describe a case of a female high school volleyball player who suffered a humeral shaft fracture while executing a floater serve. Based on the patient's history, a stress fracture was initially suspected. However, plain radiographs showed no periosteal reactions, callus formation or osteosclerosis, and thus we could not make a definite diagnosis of "stress fracture". It is suggested that an instantaneous muscle force in addition to rotational forces applied by impact with the ball caused the fracture. Her fracture healed without any subsequent disabilities based on a conservative medical management with a plaster splint, and she returned to the volleyball team. The inaccuracy of her serve form in addition to her own muscular force might be involved in the mechanism of injury. Instruction on achieving appropriate serve form might help prevent such fractures.