The transient accessibility of supply air (TASA) and transient accessibility of contaminant source (TACS) in ventilated rooms are important indices to evaluate the effect of ventilation and the indoor air quality (IAQ). These indices can be measured by experimental method or calculated with computational fluid dynamics (CFD) tools. Compared to the measurement method, the numerical method has a lot of advantages such as fast, flexible and with detailed data. In this paper, the calculation and validation of the TASA and TACS are introduced. A typical ventilated room, which is 4m long, 3m wide and 2.5m high, is taken as the example to be calculated. The values of TASA and TACS in the room are obtained by numerical simulation with two kinds of ventilation modes. The two ventilation modes are, upper-wall-supply and lower-wall-exhaust, and, floor-supply and ceiling-exhaust, which are the typical mixing ventilation and underfloor ventilation. By comparing the numerical results with the measurement data, the numerical method is validated. Furthermore, the two typical ventilation modes are evaluated by using the series indices of TASA and TACS obtained by the numerical method. It is shown that the indices are feasible of application.