The fluctuations of excess air ratios and temperatures in a combustion chamber of a spark ignition engine have been measured with the optical diagnostic technique. Excess air ratios and temperatures were estimated by the ratio of light emission intensities of 431 nm and 517 nm, 502 nm and 797 nm respectively. Data collection rate was set up at 50 kHz, which was fast enough to detect the fluctuation in one combustion cycle. Two types of fuel injectior were evaluated from the viewpoint of fluctuation of excess air ratio. Mean droplet diameter was 200 μm in Type- I injector and 110 μm in Type- II. Probability density function (PDF) showed that Type- II provided smaller variance of excess air ratio than Type- I. PDF of flame temperature was proved to be well correlated to the engine knocking. During an engine knocking, the average temperature was lower than that obtained at normal operation.