Flume experiments are often conducted to reveal the detailed behavior of debris flows. Direct sampling at the lower end of a flume has been used as a method for measuring sediment concentration in flume experiments, but measurement under non‐steady or non‐equilibrium condition is difficult. In contrast, image analysis methods can acquire spatial‐temporally high‐resolution data in a non‐contact manner and may complement conventional methods. In this study, we conducted experiments that homogenous sediment concentration field of 0～20%was made in water tank with 10 cm depth using coarse sediment with grain size of 2 mm and fine sediment with grain size of 0.2 mm. We filmed the experiments and verified the relationships between sediment concentration with image features in a fixed area. As a result, the mean and mode of pixel values for coarse sediment depend on sediment concentration up to 10%, the standard deviation and frequency of mode for fine sediment depend on sediment concentration up to 20%. Then, we analyzed the controlling factors of the image features and identified three processes: blocking of the background by sediment, darkening of the background, and changes in the brightness of the sediment. Also, we analyzed the influences of sediment grain size on image features, and it was shown that both the brightness index and the brightness uniformity index of the image were affected. In conclusion, these findings indicate that image analysis is a possible tool for clarifying both sediment concentration and grain size.