One way to teach physical education is from a developmental perspective that emphasizes the development of motor skills based on appropriateness and readiness of each student’s level (Gallahue, 1993). Knowing developmental patterns of motor skill may help teachers better observe, assess, and interpret student performances. The purpose of this study was to describe developmental patterns of arm and trunk action used in badminton game play across skill levels. The original data were collected for a statewide physical education program assessment. Eighty students (40 boys and 40 girls) from 16 high schools in a southern state were randomly selected to participate in the study. The student performances of badminton game play were videotaped and rated on 4-point scale by physical education teachers using a rubric developed by the assessment program. An observation instrument was developed to observe arm and trunk action in badminton game play in this study. Content validity and reliability of the instrument were examined. Arm action in the overhead stroke was categorized into Elbow Flexion, Elbow and Humeral Flexion, Upward Backswing, and Others. Arm action in the underhand stroke included Elbow Flexion and Downward Backswing. Trunk action in the forehand stroke was categorized into No Trunk Action, Forward-Backward Movement, Upper Trunk Rotation, Block Rotation, and Differentiated Rotation. The results indicated that for overhead stroke students at all levels used Elbow Flexion with a range from 13.9 % to 25.5 % and small percentage of students used Elbow and Humeral Flexion (2.8% to 5%). Students increased the use of Upward Backswing as their skill levels increased from 2.7% to 41.1%. In addition, students decreased the use of Elbow Flexion in the underhand stroke from 11.1% to 2.5 % as they moved from Level One to Level Four. Most students at all levels did not use any trunk movement in the games (71.5 % to 88.5 %). Small percentages of students used Forward-Backward Movement (0.3 % to 1.5 %) and Upper Trunk Rotation (0.5 % to 7.8 %). Students did not begin to use Block Rotation until they reached the level Four (1.8%). No students at any levels used Differentiated Rotation in the games. There was an increase of No Trunk Action prior to an increase in the use of Forward-Backward Movement, Upper Trunk Rotation, and Block Rotation. The findings imply that developmental patterns of arm and trunk action in badminton game play may exist. Students did not use the advanced arm and trunk action until they reached certain levels. Keyword(s): assessment, middle school issues, performance