Single crystals of the relatively new nonlinear optical material L-asparagine monohydrate have been successfully grown by the slow evaporation solution growth technique at room temperature in aqueous solution. The crystal system of the title material has been confirmed by powder X-ray diffraction. The crystalline perfection of the as-grown and annealed crystals has been evaluated by high-resolution X-ray diffraction. The as-grown single crystals (particularly when their size is large) were found to contain internal structural grain boundaries, and the crystalline perfection of these crystals was found to be improved substantially by annealing at low temperatures. The crystalline perfection and the measured physical properties were found to be correlated such that the second harmonic generation efficiency, optical transparency, fluorescence and dielectric properties are enhanced as the crystal quality improves.