Maximum rainfall intensity‐duration‐frequency (IDF) relationship are commonly used for determining the design rainfall in water resource projects. As derived from the point precipitation, the IDF formula cannot be applied on ungauged areas. In Taiwan, the network of non‐recording rain gauges has a higher density and better coverage than the recording rain gauges. This study tries to combine the rainfall characteristics in time and space abstracted from the recording and non‐recording rain gauges to develop a regional IDF relationship for ungauged areas. By using the combination of annual maximum rainfall series from 14 recording rain gauges, a dimensionless frequency curve, obtained from an EV1 probability paper, has been written as a dimensionless formula. Formulas for durations from 10 mins to 24 hrs have been derived based on the same procedures. As the parameters of the formulas have good statistical relation with average annual rainfall, which can be easily obtained at non‐recording rain gauges, a regional IDF formulas has been constructed. The regional IDF formulas have been further applied to the 99 non‐recording rain gauges to generate a regional IDF isohyetal maps for the ungauged areas in the Chia‐Nan area.