Due to high combustion efficiency and a strong tendency of auto-ignition, diesel fuel is the most important fuel. The main challenge for fluid catalytic cracking (FCC) of diesel is the conversion of heavy hydrocarbons into lower hydrocarbons. The key objective of this study is to mitigate the FCC by using zeolite-Y with SiO2/Al2O3 (40 molar ratio) and its modified forms such as H-form of zeolite-Y (HZ-Y), acid leached zeolite-Y (ALZ-Y), and acid leached H-form of zeolite-Y (ALHZ-Y) was synthesised using sol�gel method and by exchanging hydrogen ion with NH4+ and acid (HNO3). All the samples were characterised by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), gas chromatography (GC-FID) and gas chromatography with mass spectroscopy (GCMS-DB-5) to evaluate the crystal structure, bond deviations and catalytic activity. To find the lighter fractions of heavy diesel by using catalytic activities of prepared zeolite-Y and its modified forms (HZ-Y, ALZ-Y, and ALHZ-Y) were investigated by using the high-pressure autoclave reactor at 400�450 °C. The catalyst to oil ratio (2:8) was used for the cracking process, and degree of catalytic activities of the prepared zeolite-Y and its modified forms (HZ-Y, ALZ-Y, ALHZ-Y) were determined in terms of conversion yield and formation of gasoline. The conversion yield by using zeolite-Y, HZ-Y, ALZ-Y were 90.45, 15 and 14.36. Recorded zeolite-Y peaks are at 2θ values of 10, 16, 22, 30, 37, 40, 47 and 67°. The FTIR spectrum of the band from 3365 to 3489 cm�1 also called low-frequency band, belongs to the Bronsted-Lowry acid sites. In GC�MS analysis, high and low molecular weight fractions are observed. These findings will be helpful to find the new routes to produce light oil in future. These results provide the change in the morphology of various shapes of Zeolite-Y and obtained the lighter fractions of heavy diesel. Graphic Abstract: Figure not available: see fulltext. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.