A heat sink is an important passive heat exchanger in the thermal management of electronic devices or systems. The study aims to analyze the influence of the inlet velocity and heat flux on the thermal characteristics (i.e., static temperature and heat transfer coefficient) of three heat sink designs. Computational fluid dynamics (CFD) analysis is carried out for the electronic cooling process. Three heat sink designs, namely, circular pin fin, plate fin, and rectangular fin are considered in this study. The influence of the inlet velocity and heat flux on the temperature distribution and heat transfer coefficient was analyzed. The results revealed that the use of circular pin resulted the highest temperature drop when the inlet velocity increased. The highest heat transfer coefficient was observed on the circular pin fin design during the cooling process. The heat flux demonstrated a linear correlation with the temperature for all heat sink designs. The effectiveness of the heat transfer was attributed by the heat sink design and inlet velocity of the airflow. Thus, the current results indicated the thermal characteristics of the heat sink are crucially influenced by the design and velocity of airflow in the cooling process.