Abstract: As we know, Carl Friedrich Gauss was a mathematician to make use of mathematical analysis to research the number theory after Euler and Lagrange. An introduction of his study is presented systematically here. There are many exciting formulas and theorems such as the constructability of the regular 17-gon, Gaussian sum and the law of quadratic reciprocity. His main numbertheoretical work, Disquisitions Arithmeticae, and several smaller number-theoretical papers contain so many deep and technical results that Fundamental Theorem of Algebra and the ring of Guassian integers and so on. These conceptions, theorems, and formulas were all first discovered accurately by Gauss’ demonstrations. Gauss was extraordinary at converting a complex question into a simple problem. In fact, Gauss’ ideas have become more generalized. These facts are enough to prove that he had extensive and deep knowledge of his subject. A few instances represent his deep insight. Besides, we can appreciate the basic principle of methodology from Gauss’ inventions and discoveries. He never takes a process of discovery in a cover-up, and we know this from his diary which informs us about his most important discoveries.