This paper proposes a new methodology to evaluate the system operational reserve. Based on the idea of the chronological Monte Carlo, the minute basis evaluation model takes into consideration the three levels of control reserve with varying activation timeline. The proposed method calculates the probability density distributions for needed primary and secondary control reserve. Other reliability indices like EENS, LOLE can also be deduced in the model. The method is tested in IEEE RTS-96 and is applied to a real system in Southern China. The influence of different system performance on reserve requirement is investigated. The methodology reflects the transient behaviors of system from minutes to hours after a unit outage. Compared with the conventional adequacy evaluation, it is more suitable to assess operational reserve.Index Terms--Monte Carlo methods, operational reserve, power system reliability.