Since the safety and multimedia-related functions of automobile are getting increased and complex, In-vehicle network has become important. Even though FlexRay has emerged as the de-facto standard for In-vehicle network protocol, the message delay analysis problem has not been solved yet. In this thesis, we propose the probabilistic delay model of message transmission in the FlexRay dynamic segment considering the variable length of messages that share identical Frame Identifier (ID). Based on the model, we derive frame delay probability and empty minislot distribution as performance metrics, which can be used during the design phase of a FlexRay network, and also calculate transmission probabilities of additional message with new Frame ID for different lengths of it using the empty minislot probability. Finally, as model evaluation, we carry out simulation experiments with message sets that are randomly generated. Trough the experiments, we verify the models, and show that previous model only considering fixed frame length causes pessimistic delay probability compared to the proposed model. Empty minislot distribution and frame transmission probability is also analyzed for different value of system parameters of FlexRay. The delay model proposed is helpful for FlexRay system designer to determine system parameter or set frame information in DYN segment.