Abstract: Real-time tobacco diameter monitoring system was designed and implemented.The hardware parts in the system are light source, collimator system, linear CCD and processing circuits. Divergent light generating by light source was projected onto the linear CCD after getting through the cigarettes, then the level signal could be got on the linear CCD. The signal was processed by differential binary algorithm, after that the data of cigarette diameter could be calculated. Engineering prototype was produced and test results showed that the measurement accuracy of  cigarettes diameter can be reached ±15 μm.