Chapter One General Principles of the Civil Code named as Application Rules, and the article 1 in Civil Code of Taiwan defines, If there is no applicable act for a civil case, the case shall be decided according to customs. If there is no such custom, the case shall be decided according to the jurisprudence. In accordance with the provisions of article is custom treated as second source of law, to deal with civil legal relations. However, custom, in addition to the first article appeared in the civil code, but still found in other provisions (such as Article 68, 161 and 206 etc.). What means custom in civil law? What is legal effect of custom in civil Law? What is the application of custom in legal practice? In this paper is Research Focus based on a theoretical analysis and practice in the application of case about custom of article 1 und Obligation in Civil Code. Rights in Rem, Family and Succession in Civil Code are not to discuss in this paper.