Abstract A fiber optic total reflected extrinsic Fabry–Perot interferometric (TR-EFPI) sensor with a digital signal processing algorithm is developed to measure the strain of structures, such as building, bridge, aircraft, etc. Some demodulation devices had been used to sense the magnitude and direction of strain from conventional fiber optic EFPI sensors using narrow band light sources because the signal only has a sinusoidal wave pattern related to the directional change of strain. In this study, in order to measure the magnitude and the direction of strain, the sensor is simply constructed by a total reflected EFPI sensor probe and a simple digital signal processing algorithm for wave number counting without any demodulation devices. This probe is manufactured with a single mode fiber and a mirror coated fiber in a silica glass capillary tube. The output signal of this fiber optic TR-EFPI sensor was simply processed to determine the magnitude and the direction of strain. A tensile test of an aluminum beam specimen was performed by universal testing machine. In the result of this experiment, the strain from the fiber optic TR-EFPI sensor had a good agreement with the value from the electrical strain gauge.