Introduction. For testing of circumferential welds at thin-walled pipes with wall thickness lower than 6 mm using ultrasonic methods no standardization is available at the moment. Nevertheless, there are newest technologies available such as ultrasonic-phased-array technique and semi-automated handheld-scanner for pipes, with which it is possible to achieve promising and reliable results in that area. In order to bring the technique in operation in the field of coal-fired power plants or petrochemical industry a validation is needed, which will be approved by the inspecting authorities. Method. In that article we will present a successful validation of an ultrasonic-phased- array method for testing circumferential welds at thin-walled pipes with a wall thickness lower than 6 mm. Therefore, the complete geometry area was divided into several clusters, and single elements of that matrix were validated. The procedure conforms to the guideline VGB R-516 and other established standards. Besides theoretical analysis including simulations of the sound paths and reflections, extensive practical tests were performed. For this purpose six typical types of defects were analyzed by using a large number of test samples and the probability of detection was determined using this practical method. Result. The result of this work is a report for validation for each geometry cluster of the pipes including an inspection instruction and a qualification instruction for the testing equipment and the calibration samples. Conclusion. Based on that validations the ultrasonic-phased-array method was accepted by the inspecting authorities and thereby allowed to use in the regulated area of coal-fired power plants by a NDT-service provider and was applied very successful at several thousand welds.