台灣自1986年開放石化原料進口以來，關稅與非關稅進口障礙逐漸降低，而台灣中游石化業的國內廠商PCM、國內産業集中度、進口比與出口比的變化顯示四者之間可以存在聯立關係。在自由化的趨勢下，爲探討貿易行爲對本國市場結構與績效之影響，本文首先建立一開放體系之寡占模型，然後推導PCM、産業集中度、進口比與出口比間的聯立因果關係，並進而建立聯立方程廻歸模型，接蓍根據1989至1997年資料，利用3SLS對上述模型進行估計。實證結果驗證了理論模型的推導結果，並顯示：(1)PCM、産業集中度、進口比與出口比間的確存在聯立關係；(2)進口集中度對産業集中度有正向影響，但對PCM、進口比與出口比則有負向影響；(3)分散國際市場可改善PCM；(4)國內廠商間可能存在勾結關係。

Since the liberalization policy was adopted by the government in 1986, the tariffs and import restrictions of petrochemical products in Taiwan have been continuously reduced. Accompanied by these changes, domestic firms' PCM domestic concentration and import share have fluctuated significantly although export share has been continuously rising. The above fluctuation and trend in Taiwan's midstream petrochemical industries imply that there might exist simultaneous relationships among them. In order to investigate the effects of international trade on market structure and performance, causalities among domestic firms' PCM domestic concentration, import and export shares are derived based on an open-economy oligopoly model, and a simultaneous-equation system is established. The data set used in this paper consists of 21 midstream petrochemical industries. Since the data of H(superscript d) on some midstream petrochemical industries are unavailable before 1989 and after 1997, the period covered by this paper spans from 1989 to 1997, during which annual data are available for all midstream petrochemical industries under examination. Although the number of dependent and independent variables in the simultaneous-equation system is only 12, the total number of variables needed for creating these 12 variables is much more than that. Therefore, the data set is a little bit complicated, coming from 6 different sources. By utilizing the 1989-1997 data of Taiwan midstream petrochemical industries, 3SLS is used to estimate the system. The regression results confirm the causalities derived from the theoretical model, and demonstrate that there do exist simultaneous relationships among domestic firms' PCM, domestic concentration, import and export shares in Taiwan midstream petrochemical industries. Specifically, domestic concentration affects domestic firms' PCM positively while import share, export share, import concentration and country concentration of exports affect domestic firms' PCM negatively. Domestic firms' PCM import share and import concentration affect domestic concentration positively while market size affects domestic concentration negatively Domestic concentration and cost differential affect import share positively while domestic firms' PCM, export share, import concentration and capacity utilization affect import share negatively. Domestic firms' PCM, import share and import concentration affect export share negatively Based on the derived causalities, the above empirical results imply domestic firms seem to be in a situation of collusion during the period of 1989-1997, and the collusive behavior probably has originated from their subsidiary or old employer-employee relationship.