Local scientists and engineers in Thailand have recognized and pointed out a decreasing trend in discharge in the Chao Phraya river in recent years. However, no solid data or analysis to show a decline has been reported. This study is to find trends in river discharge data and to analyze significant causes of the trend. The following results were obtained:(1) As a result of examining discharge at 10 gauging stations in the Chao Phraya river basin, the significant decline of the annual discharge is found at 8 stations out of the ten;(2) 17 out of 18 sub-basins of the Chao Phraya river basin have no tendency in annual rainfall change, but the only one sub-basin in the upper Yom river basin has tendency of declining rainfall according to the Kendall rank test at 95% significance level;(3) and it is considered that major cause of the discharge decline is a change of the land use to cropland such as the orchard from paddy field rather than rainfall.