Probable maximum precipitation (PMP) is ideally the maximum precipitation depth for a given duration at a definite geographical location at a particular time in a year. The PMP is used as an input in the design of hydraulic structures. This study pointed to predict the PMP using the daily maximum rainfall recorded for 34 years (1986-2019) in Dedessa sub-basin Ethiopia, using Hershfield's statistical method. The consistence of the data checked using standard normal homogeneity test. The frequency factor (Km) value was determined using Hershfield's statistical method and the result show that for a 1-day rainfall duration, the maximum and minimum Km estimated were 3.08 and 4.34 at Dedessa and Nekemte station, respectively. The estimated maximum value of Km was 4.85 at Bedele station for 3-day duration rainfall. Using Hershfield curve rather than basin specific curve can increase the Km by 71% for 1-day rainfall duration. The majority of the stations in the sub-basin were fitted with General Extreme Value. The 1-day rainfall depth was found to vary from a minimum of 52.69 mm to a maximum of 174.3 mm at Arjo and Dedessa stations for a return period of 2 and 10,00-year rainfall, respectively. PMP isohyetal map developed using the IDW interpolation method and the result shows that the minimum PMP found at the Southeast, and Northeast of the sub-basin. However, maximum PMP was observed at the central part of the sub-basin.