This book mainly adheres to purely theoretical concepts in Thermodynamics and gives simple explanation of the laws of Thermodynamics, with clear explanation about heat, temperature, work, energy, enthalpy and entropy. This book referred to as Part I emphasis on equilibrium, pure substance, heat engine, refrigerator, heat pump, and power cycles so as to better understand enthalpy-entropy chart and psychrometric chart. This book can be useful mainly to mechanical, and aeronautical engineering students. A separate manuscript (Part II) will be published in near future, emphasizing only problem-solving approach.