Observations of flux transfer events (FTEs) at the earth magnetospause and plasmoids in the magnetotail indicate that magnetic reconnections in the earth magnetosphere are time-dependent. Motivated by the observed flux transfer events, Lee and Fu proposed a multiple X-line reconnection (MXR) model for the dayside magnetopause. The multiple X-line reconnection is intrinsically a time-dependent process and is different from the classical single X-line reconnection (SXR) process. As a consequence of MXR, plasmoids or magnetic flux ropes are intermittently formed and convected out of the reconnection region. Two- and three-dimensional MHD simulations have been carried out to study the time-dependent magnetic reconnection process. The 2-D simulation results show that the time-dependent multiple X-line process tends to occur when the system length is long and the resistivity is small. Otherwise, the quasi-steady single X-line process takes place. It is found that the occurrence of multiple reconnection sites in the current sheet may be due to either (1) the development of tearing mode instability in the local slab model, or (2) the nonuniformity of the current sheet in the global model. In the three-dimensional case, the multiple X-line reconnection process leads to the formation of magnetic flux tubes or magnetic ropes. In the three-dimensional case, the field line topology at the ends of the magnetic rope is very complicated. The simulation shows that magnetic ropes formed through the MXR process usually have “frayed” ends. In addition, a strong tube-aligned plasma flow is observed during the formation of magnetic flux tubes in the 3-D simulation.