Abstract: Credit risk theory has become one of the cutting edges in modern finance over the past few years. We investigate into one of the important issues amongst portfolio's credit risk: Copula's applications in correlated default. We discover the relationship amongst Copula and other tools for the correlated default, such as structural models and reduced form models. Additionally, different from Lando (1998), we present another method and proof for the calculation of default probability of the single firm.