In this paper, we present WS-TaaS, a Web services load testing platform built on a global platform PlanetLab. WS-TaaS enables load testing process to be simple, transparent, and as close as possible to the real running scenarios of the target services. First, we briefly introduce the base of WS-TaaS, Service4All. Second, we provide detailed analysis of the requirements of Web service load testing and present its conceptual architecture as well as algorithm design for improving resource utilization. Third, we present the implementation details of WS-TaaS. Finally, we perform the evaluation of WS-TaaS with a set of experiments based on the testing of real Web services, and the results illustrate that WS-TaaS can efficiently facilitate the whole process of Web service load testing. Especially, comparing with existing testing tools, WS-TaaS can obtain more effective and accurate test results. Copyright (c) 2014 John Wiley & Sons, Ltd.