Buletin Keairan media Informasi Kegiatan Penelitian Keairan, Vol. 2 No. 2, Desember 2009, ABSTRAK : This paper applied the M5 model Trees technique for forecasting river water levels. A water level forecasting model was built using hourly water level data at the stations Wonogiri, Colo Dam, Jurug, Karangnongko, Sekayu, Ahmad Yani, and Bojonegoro. The model was designed to forecast the river water level in the Solo River at station Bojonegaro 1, 2, 3, 4, 5, 6, 12, 18 and 24 hours ahead. The original model with zero of pruning factor showed very good accuracy, but with too many numbers of linear of linear equations. The model can be simplified so that the number of linear equtions remain small by increasing the pruning factors, but resulting in decreased accuracy. A simple model with a small number of equations but with acceptable accuracy ca be considered for use as a forecasting model.