本研究旨在透過國民小學在職教師利用學習軌道（learning trajectories）理論設計速度概念之任務，進行教學實驗，探討學生對速度概念的表現；再經由教學實驗的回溯省思，探討教師如何修正速度概念的學習軌道。研究對象為台灣中部地區公立小學一班六年級學生，利用觀察與錄影記錄師生互動內容，提供事後回溯及省思的參考，採用質性分析方式處理以下資料：課室師生對話、學生解題表現、教師省思與課後學生訪談記錄。研究發現：（1）經過學習軌道理論設計的課程與教學實驗後，學生對乘除作業判斷的時機及計算、距離與時間換算、快慢判斷、利用平均速度運算速度單位等表現良好，低分組學生對除法應用小數及分數方式尚需加強，轉換單名數的分數與描述兩單位間的關係、單位轉換存有困難，解題過程表達不甚清楚。（2）經由速度之學習軌道教學實驗後，可證實學習軌道教學模式的效果，提供學生速度概念和解題能力教學運用。研究者針對研究發現提出建議，以作未來速度單元教學或研究的參考。

This article aims to report a teaching experiment by an in-service teacher, using the principles of learning trajectories (LTs), with tasks regarding the concepts of speed and to examine the performance of students with regard to speed concepts. With the retrospective reflections made by the teacher after the teaching experiment, the instantiated teaching theories produced was discussed. The subjects of this study comprised a class of sixth-grade students at a public elementary school in Central Taiwan and their homeroom teacher. Teacher-student interactions were recorded; qualitative analyses were used to investigate teacher-student dialogue, the problem-solving performance of the students, teacher reflections, and after-class interviews with the students. Results show that: (a) after the LT-based course and teaching experiment, the students performed well in determining and calculating multiplication and division operations, converting distances and time, comparing speeds, and using average speed units. Students in the low-performing group still needed to improve on division with decimals and fractions, the conversion of fractions in simple denominate numbers, the description of relationships between two units, unit conversion, and the presentation of the problem-solving process. (b) The effect of the instantiated teaching model could be obtained after the teaching experiment of teaching students the concepts of speed and problem-solving strategies. Suggestions were then formulated for future teaching reference and research.