The second harmonic generation (SHG) characteristics of erbium doped femtosecond fiber lasers with different thicknesses of periodically poled lithium niobate crystals (PPLNs) are studied. Based on nonlinear polarization rotation mode locking and chirped pulse amplification, an erbium doped femtosecond fiber laser output with repetition rate of 100 MHz, output power of 423 mW, and pulse width of 80 femto-second (fs) is achieved at 1560 nm. Then we obtain 780 nm femtosecond lasers through SHG with three different PPLN crystals respectively. The thicknesses of PPLN crystals are 0.5, 1 and 10 mm. For 0.5 mm crystal, the SHG pulses with 104 fs width, 100.4 mW output power and 23.7% energy conversion efficiency can be obtained. For 1 mm crystal, the SHG pulses with 161 fs width,165.0 mW output power and 39% energy conversion efficiency can be obtained. For 10 mm crystal, the SHG pulses with 305 fs width, 185.5 mW output power and 43.7% energy conversion efficiency can be obtained. In addition, we explain how do energy conversion efficiency and pulse width of SHG pulses change with thickness of PPLN crystals. The experimental data provide a useful reference for the research of femtosecond optical pulses generated by mode-locked fiber laser at 780 nm band.