The dam break is a very complicated problem and it can't be foreseen exactly. Many researchers have studied the problem of dam break by predicting the behavior and the propagation of a flood wave using different approximations. In the present research, the study of dam break was performed using a mathematical modeling as a tool to predict and simulate the dam break problem. Two major mathematical models were employed namely BREACH and FLDWAV for predicting such failure parameters as well as simulating the flow routing after dam break. Derna dam (northern east coast of Libya) was taken as a case study of an earthen dam. So, collected information about Dema city and Derna dam were included in the current work. Piping failure was considered the most predictable one for Dema dam. For the current study it was shown that dam break cause a potential danger towards Dema City, since flooding occurs in the city for all hypothetical scenarios. It was recommended to predict the routing of the flow after failure due to lowering the retained water surface elevation in the reservoir and due to the use of levees and increasing walls heights in the city area.