An overload control device and a method thereof in a radio network controller are disclosed. One embodiment of the present invention includes i) a load measuring unit, measuring a load of a radio network controller every predetermined time, ii) an overload determining unit, determining whether an overload is generated by comparing the measured load with a predetermined critical value, iii) a parameter setting unit, setting a parameter of a call admission ratio according to an overload level if the overload is generated as the result of determining, the call admission ratio indicating a ratio of a maximum transmitting output as compared with an admitted transmitting output of a base station and iv) a call connection setting control unit, transmitting to the radio network controller a call connection setting signal to limit new call connection setting of a base station having a total transmitting output value which is larger than an admitted transmitting output value set by the parameter of the call admission ratio of the base station among all base stations pertained to the radio network controller.