The NH₂-MIL-125, an amine functionalized metal-organic framework, possesses good amount and strength of basic sites due to –NH₂ groups, and exhibits significant activity for base catalyzed Claisen-Schmidt condensation of acetophenone and benzaldehyde to synthesize chalcone under solvent free condition. In order to further enhance the basicity and catalytic activity of NH₂-MIL-125, the material was modified by the guanylation reaction. The guanylation of NH₂-MIL-125 using an aqueous solution of cyanamide at room temperature (30 °C) converted some of –NH₂ groups into guanidyl functionalities (16% modification) without affecting the framework structure. The guanidyl groups in the resulted material act as strong basic sites producing a stronger solid base catalyst than NH₂-MIL-125. The modified NH₂-MIL-125 exhibited enhanced catalytic activity for solvent free Claisen-Schmidt condensation reaction. The study shows that NH₂-MIL-125 and its modified form (having guanidyl groups) are potential solid base catalysts. The developed protocol for guanylation could also be useful for the modification of amine functionalized materials under green condition to strengthen their surface basic sites for enhanced catalytic performance.