Recently, optical switching and packet processing technologies have been developed and high-performance optical burst switching (OBS) networks are constructed by using these technologies. In high-performance OBS networks, several types of applications such as Grid computing and HDTV can be provided for users according to immediate reservation protocol. Because some applications require that data is transmitted reliably over high-performance OBS networks, it is indispensable to provide reliable data transfer service for high-priority users. Therefore, in this paper, we propose a reliable burst transmission method which can be available for the immediate reservation protocol. In the proposed method, both burst-cluster transmission and probabilistic pre-emption are used at edge and core nodes. By using these methods together, the reliable data transfer and the service differentiation can be provided. We evaluate by simulation the performance of the proposed method in the 14-node NSFNET. Numerical examples show that the proposed method can transmit higher-priority bursts more reliably than the conventional method while not increasing the overall burst loss probability so much. In addition, we investigate effective parameter settings from some simulation results.