A new C-alkylated flavonoid (5,7-dihydroxy-3'-(4″-acetoxy-3″-methylbutyl)-3,6,4'-trimethoxyflavone (1), along with two known C-alkylated flavonoids (5,7-dihydroxy-3'-(3-hydroxymethylbutyl)-3,6,4'-trimethoxyflavone (2), 5,7,4'-trihydroxy-3'-(3-hyroxymethylbutyl)-3,6-dimethoxyflavone (3) and two new source C-alkylated flavonoids (5,7-dihydroxy-3'-(2-hydroxy-3-methyl-3-butenyl)-3,6,4'-trimethoxyflavone (4), 5,7,4'-trihydroxy-3,6-dimethoxy-3'-isoprenyl-flavone (5) were isolated from the aerial parts of Dodonaea viscosa. The structures of all compounds were established on the basis of 1D and 2D NMR spectroscopy and mass spectrometry. The isolated compounds were evaluated for their inhibitory effect on urease and α-chymotrypsin enzyme. All the compounds (1-5) exhibited mild inhibition against urease but remained recessive in case of α-chymotrypsin.