Nanoscale HZSM-5 zeolite (20-50 nm) was hydrothermally treated with steam containing 0.8 wt.% NH3 at 773 K firstly and then was modified with mixed rare earth oxides and Ga2O3. Both the parent nanoscale HZSM-5 and the modified nanoscale HZSM-5 zeolites catalysts were characterized by TEM, XRD, IR, NH3-TPD, and XRF techniques. IR and NH3-TPD results showed that the total amount of acids sites in the parent nanoscale HZSM-5 zeolite decreased after the hydrothermal treatment, especially for the strong acid sites. The following modification using the mixed rare earth oxides had little effect on the acid properties of the hydrothermally treated nanoscale HZSM-5 catalyst; however, the further modification using Ga2O3 increased the total acid sites of nanoscale HZSM-5 catalyst, especially for the Lewis acid sites. On the other hand, the amount of total acid sites in Ga2O3 modified catalyst was still less than that in the parent HZSM-5 catalyst. The performance of the modified nanoscale HZSM-5 zeolite catalyst on olefins reduction in fluidized catalytic cracking (FCC) gasoline was investigated and it was found that the activity and stability of the modified catalyst was satisfactory, but the initial activity slightly decreased. After 300 h on stream, the olefins content in the FCC gasoline decreases from 49.6 to 15.1 vol.%. Further work showed that the ability of nanoscale HZSM-5 zeolite catalyst for aromatization reduced while its isomerization ability increased. The ratio of iso-paraffins to n-paraffins in FCC upgraded gasoline reached 8.2. The yield of upgraded gasoline reached 96 wt.%, while the research octane number (RON) was increased from 89.4 to 90.0. The components of the upgraded gasoline met the requirement of European III gasoline standard. (c) 2006 Elsevier B.V. All rights reserved.