Abstract: This paper introduces the performance of nanosized HZSM-5 and micronsized HZSM-5 in straight-run gasoline isomerization in a fixed bed under reaction conditions. The characterization of two molecular sieves were conducted by XRD, NH3–TPD and BET analysis. It is found that the nanosized HZSM-5 is more suitable for catalytic isomerization of straight-run gasoline than micronsized HZSM-5. This is because nanosized HZSM-5 has more external acid sites which favor the cyclization of paraffin and riches in secondary pores allowing more coke deposition. At 320-360 ℃ and WHSV of 1 h-1, nanosized HZSM-5 shows the best catalytic performance. Nanosized HZSM-5 can be used as an efficient catalyst for straight-run gasoline isomerization.