This Paper Addresses an Unrelated Parallel Machine Scheduling Problem with Job Sequence-Dependent Setup Times. Jobs Have Precedence Constraints. the Objective Is to Minimize the Makespan. the Problem Has Applications in Industries such as TFT-LCD, Textile Manufactures. the Problem Is NP-Hard in Strong Sense. Therefore, an Ant Colony Optimization (ACO) Algorithm Is Introduced to Solve this NP-Hard Problem. the Proposed ACO Tackles the Special Structure of the Problem. its Performance Is Evaluated by Comparing its Solutions with Cplex Method. the Results Show that ACO Outperformed the Cplex Method.