A process for preparing an electrical insulating oil comprising a reaction product having a boiling range of higher than 260.degree. C. obtained by treating a hydrocarbn feed at a reaction temperature in the range of 0.degree. to 300.degree. C. in liquid phase in the presence of an acid catalyst, said hydrocarbon feed comprising a distillate from a thermal-cracked oil obtained in a thermal cracking process for thermally cracking a petrolic heavy residual oil at a temperature not lower than 400.degree. C. and not exceeding 700.degree. C., said distillate consisting mainly of hydrocarbons boiling in the range of 120.degree. to 290.degree. C. and said distillate containing at least 30 weight percent of paraffins and at least 10 weight percent of aliphatic olefins, and separating said reaction product from the resulting mixture containing at least said reaction product and unreacted hydrocarbons.