Film cooling, along with other approaches, is well known to be an important technique for preserving the integrity of turbine blades against high temperature gases. Accordingly, continuous enhancement of film cooling performance is still one of the gas turbines community’s interests. The coolant nozzle geometry, as an important parameter controlling the film cooling behavior, has been yielded to close test in the current study seeking for the optimum one that gives best cooling performance in terms of adiabatic film effectiveness and heat transfer coefficient at blowing ratio of one. An experimentally validated model, realizable k-ε model with scalable wall function, has been utilized through the current numerical study. The racetrack slot (rectangular slot with fully round ends) was proven to outperform the typical round hole in terms of the cooling effect. The racetrack aspect ratio of seven has been designated by previous study to give the best cooling performance. Therefore, it served as a starting point for further optimization of the coolant pipe shape utilizing ANSYS Fluent Adjoint solver. The advantage of the numerical optimization tool is that it allows for irregular shape optimization. The concern that used to face irregular geometry designs is the lack of manufacturability. Nevertheless, the recent advancement in manufacturing processes, especially with the wide range and spread of additive manufacturing technique, paved the way for more powerful shape design through less constrained optimization process. Starting from the optimum racetrack geometry, the optimum irregular pipe shape was designated in two optimization steps, through which the average adiabatic film effectiveness over the test surface has increased from 0.24 to 0.34, recording a remarkable improvement.