For the purpose of getting the design data for a prestressed concrete rectangular water tank, we practiced the field strain measurement at prestressing on the structure and at loading of water pressure. Comparing the results of measurement with the values obtained from the calculations on the various models with finite element method, we could get the basic data as follows. (1) It is possible to neglect the restraint of the side wall for horizontal prestress, and also to neglect the base slab for the vertical prestress. (2) The effects of the rotation and the haunch of the wall-foot should be considered in design.