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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 the network, the message delay analysis problem has not been solved yet. In this paper, 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 analyze frame delay probability and empty minislot distribution as performance metric, which can be used during the design phase of a FlexRay network, and also derive transmission probabilities for different length of frames from the empty minislot probability. Finally, through simulation experiments, we verify the models, and show that previous model only considering fixed frame length causes pessimistic delay compared to the proposed model.