The bystander effect is a well-known phenomenon in criminology, stating that bystanders tend to inhibit people's tendency to intervene in situations where norms are violated. This paper presents an agent-based simulation model of this phenomenon. The simulation model presented demonstrates the decision process of an agent for norm violation situations with different characteristics, such as high versus low personal implications. The model has been tested by performing a number of case studies. The outcome of these case studies show that the model is able to represent the behaviour of bystanders as expected based on various experimental studies.