本研究旨在瞭解國小科學教師進行科學說明時，因果連接詞的使用情形。本研究採個案研究，研究對象為一位國小六年級自然領域教師，資料的蒐集主要為課室觀察，共觀察十六週，涵蓋三個單元。研究結果顯示，結果、條件、目的、方法、轉折等五類因果連接詞均會出現在教師的科學說明中，但不同類別因果連接詞出現的次數有所差異。在不同單元與教學型態，因果連接詞出現的類別與次數也有所不同。除了與概念多寡與難度有關之外，也與單元內容是否強調條件與變項有關。此外，教師在使用因果連接詞時，有時較為零碎片斷，或是交錯跳躍，致使呈現的因果關係含混不明。

This study aimed to understand an elementary science teacher’s use of causal conjunctions in scientific explanation, the relationship between causal conjunctions use and the features of the learning units, and the relationship between causal conjunctions use and the teaching activities. Through the qualitative methods, this study chose a teacher and his sixth grade class as the study participants. The data collected mainly were from classroom observations. Data analysis was focused on the teacher’s oral language during causal explanation. The results show that the five causal conjunctions, such as consequence, condition, purpose, manner, and concession, were shown in the teacher’s scientific explanation; however, the numbers of appearance of each category were various. Besides, the categories and the numbers of appearance of causal conjunctions were different in terms of teaching activities and the teaching units. Not only the amounts and the difficulties of concepts, but the focus on conditions and variables were related to the teacher’s use of causal conjunctions. Furthermore, when the teacher incompletely used the causal conjunctions or interpolate other causal conjunction, the cause-effect relationship became vague and hard to be understood for students.