Relationship between sound pressure in a sonochemical reactor and degradation rate of organic compounds, p-chlorophenol and p-nitrophenol, was investigated. Reaction rates were analyzed as first-order reaction. As a result, linear relationship was shown for each compound. Slope indicating the first reaction rate constant was remarkably affected by the position of the reaction flask. The sound pressure profile was measured and the relationship between the averaged pressure in a flask and the rate constant was shown. The relation was found to be remarkably different between the two. The threshold values were observed as 67kPa for p-chlorophenol and 22kPa for p-nitrophenol, respectively. From these results, it was shown that nature of compounds affects the dependency on sound pressure.