There are provided a control system and method and an engine control unit for an internal combustion engine including two types of variable valve mechanisms capable of changing a camp phase and a cam profile, respectively, which system is capable of determining an amount of fuel to be injected as a value suited to charging efficiency at the time by simplified calculation processing, thereby performing the fuel injection control properly in a simplified manner. The fuel injection control system includes an ECU. The ECU searches a map for a multiplier term Ati for a high-speed cam or low-speed cam based on the rotational speed of the engine and the cam phase, and a map for an addend term Bti for the high-speed cam or the low-speed cam based on the rotational speed of the engine and the cam phase. The ECU calculates a basic fuel injection time period Tibase as the sum of the product of an intake pipe absolute pressure PBA and the multiplier term Ati and the addend term Bti (Ati.times.PBA+Bti).