Acetonitrile aqueous solution has an upper critical solution temperature (UCST) at 272 K at c = 0.38 (c represents the mole fraction of acetonitrile). To study quantitatively how the mixing scheme of the solution changes as a function of concentration and temperature, the concentration fluctuation, the Kirkwood-Buff parameters, and individual density fluctuations for acetonitrile and water were obtained by means of small-angle X-ray scattering experiments at 273, 279, and 298 K in the range of 0.0 less than or equal to c less than or equal to 0.5. The values of concentration fluctuation became larger close to UCST and exhibit a maximum at c = 0.38. The mixing scheme of acetonitrile aqueous solution is discussed from the point of view of inhomogeneity of molecular distribution.