Disclosed is a combination process for improved hydrotreating and catalytic cracking of hydrocarbon oils, including: contacting residual oil, catalytic cracking cycle oil, and optional distillate oil with a hydrotreating catalyst under hydrotreating conditions in the presence of hydrogen followed by separation of the reaction products to obtain gas, hydrogenated naphtha, hydrogenated diesel oil, and hydrogenated tail oil; contacting the hydrogenated tail oil and optional normal catalytic cracking feedstock oil with a cracking catalyst under catalytic cracking conditions followed by separation of the reaction products to obtain dry gas, hydrogenated naphtha, liquefied petroleum gas, catalytic cracked gasoline, catalytic cracked diesel oil, and catalytic cracking cycle oil; wherein the hydrogenated tail oil and / or normal catalytic cracking feedstock oil are separated into at least two fractions, the light and the heavy fractions or normal catalytic cracking heavy feedstock oil and normal catalytic cracking light feedstock oil, prior to contacting the hydrogenated tail oil and / or normal catalytic cracking feedstock oil with the cracking catalyst. The process according to the present invention is especially suitable for conversion of hydrocarbon oils to produce more products of gasoline or diesel oil.