Ultra-low power sensors attract increasing attention as the requisite building blocks of long-life wireless sensor nodes. This work developed a proof-of-concept device to evaluate the feasibility of ultra-low power stress sensing by the leakage current of p-n junctions. Experimental results show that the variation of the leakage current of p-n junctions demonstrates excellent linearity and stability with the stress in the range from 0 to 90 MPa. When the reverse bias voltage decreases from 2.5 to 0.5 V, there is almost no deterioration of the stress sensitivity but the maximum power consumption greatly decreases from to 705 pW to 156 pW. By using the published circuit for temperature sensors, the power consumption of the stress/strain sensors based on the detection of the leakage current of p-n junctions may be decreased to lower than 1 nW. Therefore, the piezojunction effect of p-n junctions is an attractive sensing mechanism for ultra-low power stress/strain sensors.