An all-optical second-order temporal differentiator using a mechanically-induced long-period fiber grating (MI-LPFG) with a single π-shift was demonstrated. The MI-LPFG was created by pressing a fiber between two periodically grooved plates with a π-shift located at the 3/4 length from the input end of LPFG. The coupling coefficient (κ) can be adjusted by changing the pressure applied on the fiber. The experimental results show that the transfer function of the proposed MI-LPFG can be adjusted to have a transfer function as an ideal second-order differentiator. The differential performance of the designed differentiator to a Gaussian pulse is also analyzed.