In the CHMM (Continuous Hidden Markov Model) based speech recognition algorithm, Output Probability Calculation (OPC) is the most computation- intensive part. To reduce power consumption and design cost, this paper presents a custom-designed co-processor to implement OPC. The standard SRAM interface of the co-processor allows it to be controlled by various micro-controllers. The co-processor has been implemented in standard-cell based approach and manufactured in 0.18μm UMC technology. Tested with a 358-state 3-mixture 27-feature 800-word HMM, the co-processor operates at 10MHz to meet real-time requirement. The power consumption of this co-processor is 1.6mW, and the die size is 1.18mm2.