This paper proposes a robust coexistence design of MIMO radar and MIMO communication under model uncertainty. The radar waveform and the precoder of the communication system are jointly designed to minimize the total transmit power of the radar system and the communication system, while the signal-to-interference-noise-ratios of the radar system and the communication system are guaranteed with small outage probability. Simulation results show that the proposed robust coexistence system is robust against model uncertainty with the cost of using extra transmit power.