The world-wide demand for engine fuels (in Europe the demand for diesel fuel) is continuously growing. Nowadays the amount of used bio-components is increasing, too. This increase is caused by the crude oil depletion, the environmental protection and the intent to reduce the energy dependency. The mainly used alternative component in diesel fuels is the biodiesel. The performance properties of biodiesel are less favourable (poor thermal and oxidation stability, lower energy content, unfavourable cold flow properties) than the fossil diesel fuels' ones, so their blending can adversely affect the fuels properties. Thus is it important to develop and prepare another alternative fuels with better properties. A diesel fuel component with suitable properties can be obtained from waste polyolefin in several steps. The objective of our quality improving experiments was to investigate the catalytic hydrogenation of the thermal decomposition products in 5, 10, 20 and 30 % blends with gas oil over transition metal catalyst. In the frame of this work the effect of the process parameters (temperature: 300 - 360 °C, pressure: 50 bar, liquid hourly space velocity: 1.0 - 3.0 h-1, H2/hydrocarbon ratio: 450 Nm3/m3) on the quality and quantity of the liquid products was investigated.