An efficient tunable, continuous-wave, mid-infrared, intra-cavity singly resonant Nd∶YVO4/MgO∶PPLN optical parametric oscillator is presented. A laser-diode-pumped 1 064 nm laser is taken as the incident light and a MgO∶PPLN crystal is used for frequency conversion. The variation of the mid-infrared output wavelength with the grating period and temperature of nonliear crystal is investigated. The grating period of multi-grating MgO∶PPLN chip is from 29.52 to 31.59 μm by a step of 0.5 mm. The tuning wavelength of the signal light is from 1.48 to 1.63 μm and that of the idle light is 3.0 to 3.8 μm. A maximum signal power of 760 mW and idler power of 360 mW are obtained under a pump power of 8.06 W at 808 nm, with corresponding to a total optical efficiency being 13.9% when the grating period is 30.5 μm.