According to the transfer function of a fi rst-order differentiator,the condition of a uniform long period fiber grating(LPFG) as a first-order temporal optical differentiator is theoretically analyzed in this paper.The mode coupling in the periodical pressure-induced LPFG,which is formed by pressing a single mode fiber between a flat plate and a V-shaped periodically grooved plates,can be enhanced by controlling the periodical pressure applied o n the fiber,and then a large resonant loss of the LPFG can be obtained.The periodical pressure-induced LPFG in a small diameter fiber with the resonant loss larger than 50dB is experimentally demonstrated.The simulated results show that the grating is capable of completing differential operation for Gauss optical pulse with high precision.The periodical pressure-induced LPFG has potential applications in the fields of ultrafast all-optical signal processing,femtosecond optical pulse shaping,arbitrary optic al pulse generation,and so on.