Reorganizations of cyclic variations, depending on fuel type, equivalence ratio, engine load and speed, and engine geometry, are the major purposes and may cause fluctuations of output power and unburned hydro-carbon. During this study, the effects of gasoline and natural gas (NG) as fuel on cyclic variations were investigated utilizing the recorded cylinder pressure of a research SI engine over more than 400 successive cycles. This work was performed at full load, 1800rpm and compression ratio of 8 with 0.94 equivalence ratio using gasoline-air and NG-air mixtures. ُStatistical analysis of the obtained results showed that at the above conditions the coefficient of variations (COV) of indicated mean effective pressure (imep), peak pressure (Pmax) and the crank angle position of the peak pressure for gasoline-air mixture were 2.4, 1.29 and 1.04 times of those for NG-air mixture, respectively; at the optimum ignition timing, imep of gasoline-air mixture is increasing with rising Pmax and decreasing with enhancing , however, imep of NG-air mixture seems to be independent to Pmax and .