In this work, an approach to solve mixed-model assembly line balancing Problem has been suggested which consists of six stages. In the fourth stage of this approach, a heuristic method is suggested for solving the combined model. This method is programmed using C# language. This program is called “Assembly line balancing-Method of Merging Shortest and Longest Operation" symbol by (MMSLO), which is based on merging two heuristic methods “Shortest Operation time" (SOT) and “Longest Operation Time" (LOT). This approach has been applied in the actual industrial environment at Electronic Industries Company (EIC) in assembly plant for assemble three models of automatic changeover (ACH). The results of the suggested method in (MMSLO) program compared with the results of the basic method (SOT) and (LOT) that existed in"production and operations management, quantitative methods" software that symbol by (POM-QM). It has been proved the ability of the suggested method to give a better solution than the traditional methods, in which the efficiency increased from 92.75% for (LOT) method and 86.98% for (SOT) method to 93.53% for the suggested method.