A new Os-based B-site ordered double perovskite with the chemical composition of Ca2ScOsO6 was successfully synthesized. The crystal structure of the title compound was determined by employing the powder X-ray diffraction method and was found to crystallize in the monoclinic P2(1)/n space group with the cell constants of a = 5.4716(1) angstrom, b= 5.6165(1) angstrom, c = 7.8168 (1) angstrom, and beta = 89.889 (2)degrees. The temperature-dependent magnetic susceptibility data suggest that this novel S = (3)/(2) compound undergoes an antiferromagnetic transition at similar to 69 K. Fitting the high temperature susceptibility data (100-300 K) to Currie-Weisse behavior showed C = 1.734 emu.K/mol (mu(eff) = 3.72 bohr magnetons) and theta = -341 K, which is indicative of dominant antiferromagnetic interactions. Temperature-dependent specific heat measurements exhibit a lambda shape anomaly at 69 K, which is consistent with a long-range ordering of the spins. Because of a triangular arrangement of antiferromagnetically ordered magnetic ions, the system exhibits some degree of geometric magnetic frustration (GMF), but not strongly. Spin-dimer analysis, employing extended Huckel theory, reveals that a dominant exchange interaction exists (along the a crystallographic axis in perovskite layer), which violates the perfect condition for GMF.