Simultaneous measurements of the fluctuation of flow velocity, temperature, and pogitive ion concentration are made in a turbulent round jet diffusion flame. Probability density function, correlation, spatial scale, and convective velocity are obtained with the object of getting a better understanding of the structure of the turbulent diffusion flame. Main results are: (1) The structure of the outer part of the flame is characterized by intermittent appearance of high temperature, high ion concentration, and high velocity gas in nonactive, low temperature and low velocity gas. However, on the contrary, in the central part of the flame the gas parcels of the fuel or air contain reacting gases or products mixed in the molecular scale, even though the mixing is incomplete due to the large-scale turbulence and the microstructural reaction zones that exist intermittently. (2) The shape of the high temperature gas or reaction zone is stretched especially in the axial direction. The difference of the profiles of scales obtained from the fluctuations of velocity, temperature, and ion concentration is noticeable. (3) Cross correlation and probability density function of temperature and velocity are intimately related to their time-averaged profiles, and their relations are qualitatively comprehensible if we consider that gas parcels exchange positions laterally by turbulent mixing while retaining their original temperature and velocity before the exchange as is presumed in the mixing length hypothesis. (4) Convective velocity of temperature fluctuation U-TC is nearly equal to the time-averaged flow velocity U, but UTC is a little less than U at the central part of the flame and a little larger than U at the outer part. The profile of the convective velocity of the ion concentration fluctuation Uic is more flattened as compared with/~.