Reaction of dibenzalacetone with hydrazine hydrate and formic acid yielded a novel 2-pyrazoline 1 , characterized by ESI-MS, FT-IR, UV, 1 HNMR and 13 CNMR data and microanalysis. Keywords: dibenzalacetone; hydrazine hydrate; pyrazoline; 5-phenyl-3-[( E )-2-phenyl-vinyl]-4,5-dihydro-1 H -pyrazole-1-carbaldehyde; fluorescence Introduction Substituted pyrazolines are useful in pharmaceutical and agrochemical research. Pyrazoline derivatives with a phenyl group at the 5-position possess good film-forming properties, exhibit excellent characteristics of blue photoluminescence and electroluminescence [1]. Pyrazolines are also used as optical brighteners and whiteners. Pyrazolines display various biological activities such as antimicrobial [2], antifungal [3], antidepressant [4], immunosuppressive [5], anticonvulsant [6], anti-tumor [7], antiamoebic [8], antibacterial [9] and antiinflammatory [10] activities. Syntheses of pyrazolines by reaction of α,β-unsaturated carbonyl compounds with diazoalkanes [11] or with hydrazine hydrate [12,13] were reported in literature. N-Substituted thiocarbamoyl-3,5-diphenyl-2-pyrazoline derivatives having NS donor, their palladium II complexes [14] and hydroxyl phenyl pyrazolines have also been synthesized [15]. The reaction of