A novel metal-organic coordination complex nonlinear optical crystals, tri-allylthiourea cadmium chloride [(CdCl 2 (AT) 3 ] and tri-allylthiourea mercury chloride [(HgCl 2 (AT) 3 ] abbreviated as ATCC, ATMC (AT is Allylthiourea i.e.,CH 2 =CHCH 2 NHCSNH 2 ) has been synthesized and grown as single crystals. It was synthesized in deionised water and further recrystallized to improve its purity. Single crystals of the allylthiourea co-ordination complex nonlinear optical crystals tri allylthiourea cadmium chloride (ATCC) with dimensions of 14x14x10 mm 3 and tri allylthiourea mercury chloride (ATMC) with dimensions of 15x15x12 mm 3 were grown successfully from aqueous solution by solvent evaporation as well as by temperature lowering method. It exhibits powder SHG efficiencies higher than that of a well known organic NLO crystal Urea. The solubility of the as grown crystals was estimated from the aqueous solution and the effect of different metal ions on the grown crystals, structural, thermal, spectral and optical properties were analyzed. XRD studies the reveals the same structure of both materials. Influence of the different central metal (Cd and Hg) atoms, changing the thermal properties of the materials when NLO complexes formed with the common ligand allylthiourea . The metal co-ordination was confirmed form the spectroscopic analysis. From the UV transmittance studies, red shift was from the transparency cut-off wavelengths. The value is 285nm for ATCC is and is 335nm ATMC, Non-linear an optical study confirms the suitabilities of the as grown crystals for the non linear optical applications.