Ubiquitous computing enables computational services pervasive. Web service is an efficient technology to provide interoperability between components dispersed on networks and various devices, regardless of platforms and languages, and thus, is massively used to develop ubiquitous computing applications. In order to provide transparent services in ubiquitous environment, we need to consider various quality constraints during execution of web services, selection of contexts to use, and determination of operational devices. In this paper, we define a quality model for ubiquitous computing applications, and propose a quality-driven web service composition methodology. EWC (Event-driven Web service Composer) is our tool for supporting the proposed methodology. We illustrate how users can benefit from EWC to provide ubiquitous services transparently, with an example of monitoring diabetics.