Novel Ga-MCM-41-SO3H is designed as a sulfonated mesoporous hybrid organic-inorganic catalystand is characterized by various modern techniques. The catalyst was used in the synthesis of various heterocyclic phenyl-[1,2,4]-triazolidines through the reaction of aldehydes, urazole, and dimedone. Moreover, the catalyst was stable and reused at least three times without loss of catalytic activity.