To meet the challenges of the crude oil becoming heavier and poorer quality, and the tendency of the increasing liquefied petroleum gas (LPG) and diesel in fluid catalytic cracking (FCC) process, the ultrastable Y (USY) zeolites were further modified by citric acid to optimize their nanopores and acid sites. The nanopores and acid sites of modified zeolites were extensively investigated by N-2 adsorption, XRD and in situ IR methods. The micro-activities of modified samples were also evaluated. The results showed that the nanopore volume of the modified samples is rather more developed than that of the parent zeolite. It indicated that micropores and mesopores might be generated by skeletal reconstruction. The dealumination process might be occurred gradually from the upmost surface to subsurface, then to bulk sites of the USY framework because of the larger volume of citric acid molecules. It resulted in the gradient distribution of nanopores and acid sites in modified zeolites. The hydroxyls with IR bands of 3610 and 3575 cm(-1) in the micropores or small cages bears the stronger acidity and much more sites than those in mesopores or larger cavities. This gradient acid distribution was rather preferable to produce preferably LPG and diesel in FCC process. MAT evaluations indicated that the LPG and diesel yields of citric modified samples were effectively improved. Optimization of nanopores and acid sites of ultrastabilized USY zeolites modified with citric acid in unbuffered system might result in an industrial process to design novel FCC catalysts.