Cycle-to-cycle fluctuation in combustion of a lean mixture and a nitrogen-diluted mixture in a spark-ignition engine was investigated under the conditions that the fluctuations of mixture strength and charge quantity were very small. Pressure history in the cylinder was measured, and indicated mean effective pressure, rate of heat release and combustion duration were determined. The experiment was carried out under various conditions of engine speed, swirl ratio, equivalence ratio, degree of dilution of intake charge, ignition timing, and spark location. As a result, it was found that the correlation between the duration of early-stage combustion in crank angle θ(00-05) and the indicated mean effective pressure P_mibecomes strong, under the conditions of lean or diluted mixture, but the correlation between the two parameters is not recognized under the condition of a near stoichiometric mixture.