Polyolefin pyrolysis in open or closed systems were compared in terms of the differences in the carbon number distribution of the produced wax and oil to better understand the product formation mechanism for the two processes. The pyrolysis was conducted in a small tube reactor either in a closed mode or in an open mode at a temperature range from 400oC to 500oC. In the closed mode, the sample was sealed in the reactor. In the open pyrolysis mode, the pyrolysis vapour was continuously carried out of the reactor by nitrogen and then condensed. The carbon number distribution of the wax and oil was measured using simulated distillation. In the open pyrolysis mode, the carbon number distribution did not vary significantly by varying pyrolysis temperature, and the distribution agreed with the prediction of random scission model of free radical depolymerisation. This is mainly attributed to the evaporation effect in the open reactor, which reduced the chances of secondary reactions. In the closed mode, the carbon number distribution had a significant change with pyrolysis temperature and time, and it did not obey the random scission model, indicating the existence of secondary reactions.