In this paper, a stress sensor using piezoelectric element is developed to monitor the feature of stress history of structural member. The sensor is a rectangular box (90mm long, 60mm width and 25mm thick) made of thin aluminum plate, in which piezoelectric element, electric circuit including integrating circuit, button type battery, make and break switch, stress level meter using light emitting diodes (LED) and stress level cross counter are facilitated. The stress level meter can turn on and off different color diodes according to the change of stress level. The stress level cross counter can display the number of cycles which exceeds a specific stress level during a monitoring period. The sensor is bonded onto the structural member by using adhesive agent. The monitoring period can be determined by the attached switch. In order to see the performance of the prototype sensor, experiments are carried out using a fatigue testing machine. The sensor is bonded onto a smooth specimen and multiple repeated stresses are applied to the specimen. The signal of the LED and the output of the counter are investigated by changing wave shape, stress amplitude and frequency. Followings are made clear from the experiments. 1) The sensor can operate under the stress range above 10 MPa, and the frequency from 0.05 Hz to 5 Hz. 2) LED of the stress level meter can turn on and off accurately under any stress amplitude and frequency. 3) When the stress frequency is 0.5 Hz or less, the level cross counter can accurately count the number of cycles which exceed the predetermined stress level.