Organic farming technology may be relatively unknown to farmers at the time when they switch from conventional into organic farming. Therefore, experience gained over time and learning by doing may be important determinants in the efficiency of organic farming. It may also take time to reach the optimal nutrient stock of soil and optimal nutrient supply for arable crops under organic farming. Thus, efficiency of organic farming can either grow or decrease over time depending on the nature of the technology and the learning process. This paper estimates technical efficiency of organic farming and its development over time. We control for possible selection bias and regional heterogeneity when estimating a stochastic frontier distance functions for a sample of conventional and organic dairy farms in Finland. The results suggest that organic dairy farms are less technically efficient than conventional farms. Technical efficiency at first diminishes when the conversion towards organic production starts. After 6 years from the switch, technical efficiency starts to increase again. The estimates signal that the length of the conversion and learning process of organic farming is in average 6-7 years.