We have studied the influence of out-of-plane strain on the electronic properties of zigzag graphene nanoribbons (ZGNRs) using first principles methods. We find that the electronic properties of ZGNRs are highly sensitive to the strains. As the strain increases, the band gap first remains almost invariant and then decreases monotonically, which could be used to design high precision strain sensors. ZGNRs based sensors would possess higher sensitivity and quicker response compared to the conventional sensors.