The tar problems are the major limit for development of biomass gasification. Biomass char has been proven to be an economical and effective catalyst of tar destruction for both utilizations inside the gasifier and in downstream processes after the gasifier. In order to investigate the mechanism of catalytic cracking of tar over a biomass char bed, experimental research was performed in a bench-scale tube flow reactor, choosing rice straw char as the catalyst bed, naphthalene as the model tar compound, and argon as the inert atmosphere. The effects of temperature (700-1000 degrees C), tar concentration, time on stream (0-330 mm), presence of syngas, and pretreatment of char (treated with deionized water or Ni(NO3)(2) solution) on tar conversion were evaluated. The variation of the inner pore structure of biomass char during the process of tar removal was also investigated. Results showed that the original biomass char exhibited good catalytic activity in tar cracking and better stability compared with Ni(NO3)(2) pretreated char. However, because of the naphthalene cracking reaction, soot was formed on the active sites of the inner pore surface of the char, leading to the deactivation of the char. A rapid decline in specific surface area of the char was observed from 262 to 4.6 m(2)/g when the test had begun to run for 5 mm with high tar concentration (25 g/Nm(3)) and a temperature of 800 degrees C. The presence of syngas in the atmosphere could slow the process of deactivation of char.