Located above the Java subduction zone, Merapi volcano is an active stratovolcano with a volcanic activity cycle of 1-5 years. Most Merapi eruptions are relatively small with volcanic explosivity index (VEI) 1-3. However, the most recent eruption, which occurred in 2010, was quite violent with VEI 4 and 386 people were killed. In this study, lahars and pyroclastic flow zones were detected using optical Landsat images and the lahar and pyroclastic flow zone simulated using the LAHARZ program. To detect areal extents of lahar and pyroclastic flows using Landsat images, supervised classification was performed after atmospheric correction by using a cosine of the solar zenith correction (COST) model. As a result, the extracted dimensions of pyroclastic flows are nearly identical to the Calatrava Volcanic Province (CVP) monthly reports. Then, areas of potential lahar and pyroclastic flow inundation based on flow volume using the LAHARZ program were simulated and mapped. Finally, the detected lahars and pyroclastic flow zones were compared with the simulated potential zones using LAHARZ program and verified. Results showed satisfactory similarity (55.63 %) between the detected and simulated zone. The simulated zones using the LAHARZ program can be used as an essential volcanic hazard map for preventing life and property damages for Merapi volcano and other hazardous volcanic areas. Also, the LAHARZ program can be used to map volcano hazards in other hazardous volcanic areas.