Boiler control system includes a first device for measuring or computing a rate of change in steam temperature and a generated thermal stress in a pressure part of a boiler. The second device is used for controlling steam temperature or a rate of change in the steam temperature at the pressure part of the boiler in accordance with a desired value. The third device which has the function of storing in memory combinations of steam temperature and thermal stress at each moment is obtained by the first device. The third device determining, from the combinations stored in the memory, a relationship between a steam temperature changing rate and a maximum thermal stress caused by the steam temperature chaning rate, determines, in accordance with the relationship, a steam temperature changing rate limit value necessary for maintaining an actual thermal stress in the pressure part of the boiler below a maximum thermal stress limit value.