This paper reports a passively mode locked fiber laser from a linear cavity. It consists of semiconductor saturable absorption mirror (SESAM) and fiber Bragg grating. Another function of the SESAM is to achieve passive mode locking based on saturable absorption. The polarization of picosecond pulses is maintained by polarization maintaining fiber. All-fiber structure of fiber laser is achieved by a homemade fiber coupled SESAM. Repetition rate of mode locked trains of 40 MHz, pulse width of 25 ps, spectral width of 0.12 nm, mode locked threshold average laser power of 8 mW, and the maximum laser power of 19 mW are obtained. Mode locked train without Q-switched mode locking is proved by radio frequency (RF) spectrometer.