A prediction device and method for use in a Viterbi decoder is provided. The prediction device is applicable to a communication system with low bit error rate for reducing the count of accessing path memories, thereby lowering the power consumption of the system. The prediction device needs not activate the traceback modules when making a successful prediction. In other words, no access to the path memories is required. The predicted bits decoded and outputted by the decode bit registers are the decoded bits from the Viterbi decoder. Therefore, the prediction device saves much traceback works and power consumption for decoding.