The start-up stage of tokamaks includes complex phenomena such as gas breakdown, atomic processes, runaway electron production, etc. The volume-averaged equations are used for a zero-dimensional transport study on the start-up phase of the KAIST-tokamak. This set of nonlinear equations includes the impurity effects such as oxygen line radiation and ionization, etc. The temporal evolution of the plasma density, temperature, and current are obtained by solving the equations numerically. From these results, we analyze different regimes and obtain the important parameters for a so-called ''good discharge''.