Enabled by Service-Oriented Architecture (SOA), recently Software as a Service (SaaS) and Cloud computing are gaining momentum in the industry. An open issue is how to ensure accountability in business services offered through Internet. Traditionally a contract is an effective legal means to uphold accountability in business transactions. In this paper, we propose a novel service contract model called OWL-SC for e-Services. Based on OWL-DL and SWRL, OWL-SC model can be used to disclose obligations of both e-Services consumer and e-Services provider. More importantly, the model allows service participants to monitor the service contract execution and keep track of obligation fulfillment for each party during service delivery. We also propose a graphical model SC-CPN based on Colored Petri-Nets (CPN) to formally model contract obligations and their interdependencies. SC-CPN can also be used to validate the correctness of obligations in OWL-SC through simulation and state space analysis. Finally, we use the Congo Book service as an example to illustrate how to use OWL-SC and SC-CPN to build a service contract model.