Analyses of stable hydrogen and oxygen isotopes, ²H and ¹⁸O, were conducted on 48 water samples collected during September and October, 1997. The water samples included 3 rain water samples, 10 river water samples, and 35 groundwater samples within the Lampang basin. An assessment of groundwater origin was made on water samples collected from three aquifer units. These three aquifer units are Holocene unconsolidated sediments, Pleistocene unconsolidated sediments, and Tertiary unconsolidated-semiconsolidated sediments. Isotopic analyses of the groundwater samples showed no significant difference in d ²H and d ¹⁸O, suggesting that the groundwater in the three aquifer units has the same origin. The groundwater data points on an d ²H and d ¹⁸O diagram deviated from the global meteoric water line and fall along the evaporation line in the range between d ²H and d ¹⁸O values of rain water and river water. This indicated that groundwater in the Lampang basin is directly recharge from local precipitation and is certainly affected by evaporation.