In this article, a novel model of OFDM system in the frequency domain based on network calculus is introduced. The proposed model employs modular concatenation of selected network calculus components to represent the effect of subcarriers of OFDM system on the QoS performance. By the formulas of the proposed model, the minimum transmit power and the total number of error bits can be calculated and the maximum transmit rate on each subcarrier can also be evaluated. With a practical example, the effectiveness of the proposed model is demonstrated. It can be seen that our model offers an efficient way of deriving minimum transmit power and evaluating QoS performance of OFDM system owing to the powerful analytical strength of network calculus.