A process for producing middle distillates by hydrocracking a heavy distillate oil under middle pressure uses a hydrofining catalyst and a hydrocracking catalyst in series and once-through operation. The once-through volume conversion to 350.degree. C.--products is up to 80 vol. %. The yield and selectivity of middle distillates are greater than 40 wt % and less than 50 wt %, respectively. The process is particularly suited to treat heavy distillate oil having the nitrogen content of up to 1500 ppmw, the sulfur content of up to 3.5 wt % and dry point in the boiling range of higher than 500.degree. C. to produce the feed for reforming process, jet fuel, the low sulfur content diesel, the feed for catalytic cracker and the feed for producing ethylene.