透過一開放經濟體系寡占模型的建立與推導,以民國76-80年台灣中游石化業資料為依據,用完全訊息最大概似法對內銷PCM、進口比與產業集中度三條聯立方程式進行估計,本文發現,產業集中度與國內需求成長率對內銷PCM的影響為正面的,且具有統計顯著性。內銷PCM、關稅與運輸成本對進口比的影響皆是負的,且具有1%統計顯著性;產業集中度與成本差異對進口比則有顯著的負面影響。內銷PCM對產業集中度的影響顯著為負的,進口比與產能利用率對產業集中度具有顯著正面影響,而市場規模對產業集中度則具有負的非線性影響;最後,內銷PCM或進口比與產業集中度間皆存在顯著直接聯立關係。 Based on 1987-1991 data of Taiwan's midstream petrochemical industries, and applying the full information maximum likelihood method to a simultaneous equations model of domestic PCM, import ratio and industry concentration derived from an oligopoly model in a small open economy, this paper finds that domestic PCM is directly and significantly related to concentration and the growth rate of domestic sales. Domestic PCM, tariffs and transportation costs affect import ratio adversely at the 1% level of significance, while the latter is positively and significantly related to concentration and cost differentials. Domestic PCM affects concentration adversely at the 5% level of significance, while the latter is positively and significantly correlated with import ratio and the degree of capacity utilization. Furthermore, concentration has a negatively and nonlinear relationship with market size. Finally, there exist direct simultaneous relationships between concentration and domestic PCM (import ratio).