Engineering Thermodynamics is a core course for students majoring in Mechanical and Aerospace Engineering. Before taking this course, students usually have learned Engineering Mechanics-Statics and Dynamics, and they are used to solving problems with calculus and differential equations. Unfortunately, these approaches do not apply for Thermodynamics. Instead, they have to rely on many data tables and graphs to solve problems. In addition, many concepts are hard to understand, such as entropy. Therefore, most students feel very frustrated while taking this course. The key concept in Engineering Thermodynamics is state-properties: If one knows two properties, the state can be determined, as well as the other four properties. Unlike most textbooks, the first two chapters of this book introduce thermodynamic properties and laws with the ideal gas model, where equations can be engaged. In this way, students can employ their familiar approaches, and thus can understand them much better. In order to help students understand entropy in depth, interpretation with statistical physics is introduced. Chapters 3 and 4 discuss control-mass and control-volume processes with general fluids, where the data tables are used to solve problems. Chapter 5 covers a few advanced topics, which can also help students understand the concepts in thermodynamics from a broader perspective.