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The percent of total runoff as surface runoff is an indicator of soil structure and functioning of subsurface drainage system in clay soils. Water logging due to low infiltration and low hydraulic conductivity of wet soil increases the risk of surface runoff, and thereby the risk of soil erosion and phosphorus leaching. Lowering groundwater table by efficient subsurface drainage has been found to enhance physical and biological processes that improve the structure of clayey soils. The effects of drainage improvement on surface runoff and soil physical properties have seldom been reported. We have studied the long-term effects of subsurface drainage improvement on soil physical properties and surface runoff on a heavy clay soil under boreal conditions. The runoff determinations were carried out for five years before and nine years after the drainage improvement. During the study period, the soil was autumn ploughed annually to 20 cm depth and spring cereals were grown. Before the drainage improvement, surface runoff constituted 6080% of the total runoff but it declined to 1040% after improvement. Mean values of macroporosity and saturated hydraulic conductivity for subsoil (2060 cm layer) measured ten years after drainage improvement were higher than the mean values measured two years before drainage improvement, indicating that the processes relevant to the formation of clay soil structure were enhanced.