An electronic control apparatus for vehicles has at least two microcomputers, each having a non-volatile flash memory which stores a vehicle control program and data. Each microcomputer is reset when an abnormality is detected. Each microcomputer is applied with an identification signal indicative of a main one or sub one. When a data rewriting instruction is applied from an external writing device, one microcomputer which receives the identification signal indicative of the main one operates to release the other microcomputer from the reset condition after an elapse of a delay time period. A new control program and data transmitted from the external writing device is written into the corresponding flash memory in place of the previously stored control program and data.