A new three-dimensional microporous metal-organic framework Cu(BDC-OH)(4,4'-bipy)center dot G(x) (UTSA-15; H2BDC-OH = 2-hydroxy-benzenedicarboxylic acid, 4,4'-bipy=4, 4'-bipyridine, G = guest molecules) with functional -OH groups on the pore surfaces was solvothermally synthesized and structurally characterized. UTSA-15 features a three-dimensional structure having 2D intercrossed channels of about 4.1 x 7.8 and 3.7 x 5.1 angstrom(2), respectively. The small pores and the functional -OH groups on the pore surfaces within the activated UTSA-15a have enabled their strong interactions with CO2 and C2H2 which have been revealed in their large adsorption enthalpies of 39.5 and 40.6 kJ/mol, respectively, highlighting UTSA-15a as the highly selective microporous metal-organic framework for the CO2/CH4 and C2H2/CH4 gas separation with separation selectivity of 24.2 and 55.6, respectively, at 296 K.