This project aims to analyze fleet heterogeneity and its relationship to airlines' use of leasing strategies around the globe. Using data collected from Cirium, we develop Blau's index of fleet heterogeneity for both aircraft frame and engine at the manufacturer, family, and model level, covering the top 200 passenger airlines worldwide. The study covers 18,808 in-service aircraft at an average age of 11 years, including 201 aircraft models and 247 different engine types. The average leasing rate is about 70% among the sample airlines. In this study, we will dive into the fleet composition between different manufacturers and their percentage distribution across regions such as Europe, Africa, Asia-Pacific, North America, and South America. In addition, we will compare the reliance on aircraft leasing by airlines in different regions and in adopting different business models. We will estimate the relationship between fleet heterogeneity and leasing strategy and test our hypothesis about the role played by leasing strategy in helping airlines diversify their fleet and expand their market share rapidly. The trend of low-cost carriers using a more standardized fleet will also be examined. The findings from our study will provide management implications for airlines and shed light on the importance of strategically integrating aircraft financing and fleet composition decisions.