In Taiwan, prior to the petroleum industry liberalization starting from 1990, the whole petroleum market was highly regulated and monopoly. Since 90s, the first private owned gas station was built in 1997; the Formosa Petrochemical Corporation went public in September, 2000; the Petroleum Administration Law was implemented on October 13th, 2001; and almost all famous international petroleum businesses had plans to establish branches in Taiwan. As the result, the monopoly market gradually became more and more competitive during that period. The China Petroleum Corporation, one of the state-owned enterprises, was the only price leader in the market, and currently still has absolute influence. This research, therefore, is focus on how CPC adjusting its business strategies and marketing management while facing the intensive competition in this liberalizing market. The main findings are as follows: 1. Since the Formosa Petrochemical Corporation and other international petroleum companies joined into this competitive market, CPC has actively adopted much adjustment. However, the company and all staffs must try harder if they plan to keep 70% market share. 2. For future competition, CPC has adopted not only some defensive tactics to keep the current market shares, but also many aggressive strategies so that the company can continuously penetrate, expand, and diversify the new market . Moreover, CPC has asked its staffs to provide their best service in order to secure the client base and to improve the competence. In fact, those good business strategies really helped CPC increase its total sales and profits; or keep its current market shares at least. However, CPC still need to deploy some other marketing approaches such as to keep its cost down, to better off its service, and to train qualified staffs, so that the company can advance its competitive preference and diversify its marketing strategies as well. Keywords: petroleum industry liberalization, the Petroleum Administration Law, business strategies, and market share