Plate fin heat exchangers are often used for air conditioner. In order to obtain better performance, flat and multi-hole type tube is used to be adoptive as a refrigerant tube rather than normal cylindrical tube. However connecting method between the flat tube and cooling fin is not realized for practical application. This report investigates a novel method of connection by giving inside pressure and plastic deformation to a special flat tubes which has doglegged thin inner partitions. Elasto-plastic analysis by Finite Element Method is performed to investigate behavior of the flat tube under some conditions.