A core-cladding waveguide structure of the a-axis, Nd:MgO:LiNbO3 single crystal fiber with step refractive index profile is achieved by a Mg-ion indiffusion process. A laser diode pumped Nd:MgO:LiNbO3 single crystal fiber extracavity frequency doubled laser at room temperature has been developed for the first time. A CW green laser of 10 μW (at 0.532 μm) is obtained under 19 mW (at 1.064 μm) input power.