Efficiency improves and pollutant emissions decrease of the automotive diesel engine can be assured by Liquid Petroleum Gas fuelling. Paper target is the study of cycle variability at LPG fuelling, which shows new, important, considerable implication and effects of LPG use at diesel engine, completing the general experimental investigation of engine running at fuelling with alternative fuel for a clean environmental. The K9K EURO 4 diesel engine was mounted on the test bed, equipped with instrumentation and fuelled with LPG by diesel gas method. The investigated engine operating regimes were 55% and 70% load at speed of 2000 rpm, at fuelling with diesel fuel and LPG at different substitute ratios. The increase of LPG cycle dose leads to the increase of cycle maximum pressure and maximum pressure cycle dispersion, thus the dispersion of successive running cycle’s increases. This fact may become a limitation criterion for the LPG substitute ratio value.