Mastery in fundamental movement skill (FMS) performance (e.g., kicking, jumping, throwing) has been considered an important factor in preventing unhealthy weight gain (Okely, Booth & Chey, 2004); as well as helping increases in participation of organized and habitual physical activity (Foley, Harvey, Chun & Kim, 2008; Hume, Okely, Bagley, Telford, Booth, Crawford & Salmon, 2008; Mazzardo, 2008; Okely, Booth & Chey, 2004; Okely, Booth & Patterson, 2001) among children and adolescents. Thus, assessing FMS development becomes crucial in school settings. Therefore, the purpose of this study was to develop and collect initial validity evidence for a new observational assessment tool (FG-COMPASS) to evaluate FMS development of children 5- to 10 years of age. In Phase I of this study, 110 video clips of children performing 5 locomotor and 6 object-control FMS were developed. In Phase II, the rating scales (composite decision trees) were developed for each FMS. In addition, the efficacy of the decision trees was evaluated by comparing judgments of 30 undergraduate students with a standard. Weighted kappa indicated that the agreement was best for hop (K w= .85), followed by strike and batting (Kw= .79), skip (Kw= .77), overhand throw ( Kw= .74), catch and hand dribble (Kw= .72), and horizontal jump (Kw= .70). The poorest agreement occurred in the skills of kick (Kw= .51), and side slide and leap (Kw= .61). The proportion of specific agreement (PS) was calculated for each skill with the purpose to find out the source of disagreement. Skills that had at least one category (e.g., initial, elementary, mature) with PS values below .70 were further inspected. Six skills were selected for further analysis (side slide, horizontal jump, leap, kick, hand dribble, and overhand throw). The decision trees for all six skills underwent modifications. In conclusion, this study provided initial validity evidence that the decision trees (rating scale) developed for the FG-COMPASS could be used to classify individuals based on their FMS development. However, reliability and objectivity studies need to be conducted to test the feasibility of this instrument when used in the field.