In this talk, we will talk about a classical problem related to the prime number theorem: the difference of /pi(x)-li(x), and introduce a so-called Skewes' Number. Here /pi(x) is the number of primes up to x and li(x) is a integral of (log t)^{-1} from 0 to x. These two objects are studied intensively by Bernhard Riemann in his famous memoir and play important roles in Riemann Hypothesis.