Solid oxide fuel cells (SOFCs) consist of ceramic materials. Because of the brittle nature of ceramics, durability decrease of a SOFC system or mechanical failure of cells can be caused by transient behavior, that is, sudden temperature variation and axial temperature gradient. Therefore, it is important to understand the transient behavior of a SOFC. To study the transient behavior of a direct internal reforming (DIR) planar solid oxide fuel cell (SOFC), a one-dimensional dynamic model is presented. This model is modified to predict the heat-up and start-up behavior. The heat-up time and start-up time are calculated from the model. The heat-up time can be adjusted by manipulating air velocity and temperature. During the start-up mode, the effects of initial temperature of the PEN (positive electrode/electrolyte/negative electrode) structure and air temperature are investigated. The fuel cell characteristics such as cell voltage, current density distribution, and temperature distribution can be calculated from the modified dynamic model. Consequently, this model can be useful to investigate the transient behavior during heat-up and start-up modes.