The conventional derivation of the expression for inductance of power transmission lines given in books is more a recipe than a derivation starting from a law of physics. The serious shortcomings of the traditional approach are that the definition of flux linkage lacks strong connection to Faraday's law, and unnecessary assumptions are made. This paper gives a complete rigorous derivation of the expression for inductance of power transmission lines. This paper will help instructors as well as students, since it addresses the deficiencies present in the derivation given in textbooks.