Cost overrun is a common phenomenon observed in construction projects worldwide. It is one of the biggest challenges experienced in the construction industry that leads to overstretched budgets, and directly affects the country's gross domestic product (GDP). This study aims to identify the factors that significantly contribute to cost overrun for road network projects in Egypt during the implementation phase. A survey was conducted to determine the most critical factors affecting the cost overrun for road network construction projects. Results showed that the main factors causing cost overrun are inaccurate cost estimates, design modifications, quantity changes, variation orders, political interference, inflation, specification changes, and change in the scope of work. This study will help decision-makers identify the factors that may affect the costs of future road projects and provide guidelines to mitigate the adverse effects observed during the implementation stage.