Catalytic activity of commercial ruthenium catalyst (ruthenium (5%) supported on alumina (Ru5%Al2O3)) on renewable hydrocarbon production from stearic and oleic acid via hydrotreatment was investigated. Moreover, the effect of Ru5%/Al2O3 on aromatic (ethylbenzene) and saturated cyclic hydrocarbon (cyclohexane) structure modification under the same reaction conditions was determined. Full conversion of fatty acid feedstock into pure hydrocarbons were achieved over ruthenium catalyst. Linear alkanes (n-pentane to n-octadecane) were produced with liquid hydrocarbon yields between 54% to 66%. Main products from both feedstocks were n-pentadecane to n-heptadecane. Ru5%/Al2O3 also exhibited effect on structure modification of ethylbenzene and cyclohexane under hydrotreatment conditions. Complete hydrogenation of ethylbenzene occurred and ethylcyclohexane as main product was obtained. The effect of ruthenium catalyst on ring opening reactions of ethylcyclohexane and cyclohexane was observed. Investigated catalyst Ru5%/Al2O3 shows decent performance on renewable hydrocarbon production from free fatty acid feedstock and hydrogenation of double bonds under hydrotreatment conditions using low catalyst amount and relatively short residence time. The effect of Ru catalyst on ring opening reactions of cyclic compounds and cleavage of -C-C- bonds of linear hydrocarbon chains has potential for practical application in structure modification. Hydrocarbon structure modification can be applied for synthesis of wide range of linear and branched hydrocarbons from alternative and fossil sources.