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Modularity is one of the prominent principles in designing services which must be adhered during service design. Services can be simply defined as modular units which concentrate on a single business functionality. Focusing on a single business functionality, modular services can be easily reused in various contexts and also can be composed to satisfy new requirements. On the other hand, modularity in services prevents the propagation of changes to other services and thus simplifies maintenance of service-oriented systems. Since service-oriented analysis and design methods lack on providing a quantitative model for service modularity level evaluation, identification of optimally granular services is the key challenge in developing service-oriented solutions. Thus, it is necessary to define metrics to measure the degree of service modularity prior to the implementation, a concern rarely noticed in previous researches. In this paper, we propose a metric suite to measure the degree of modularity of a service at the design level from conceptual point of view. More precisely, using existing semantics in business processes, the proposed metrics evaluate modularity of a service considering three known aspects, i.e., decomposability, composability and understandability. Finally, a case study is used to explain our metrics and demonstrate their effectiveness.