Objective. The purpose is to validate and discuss the possibilities to use the Motorisk Utveckling som Grund för Inlärning (Motor skills Development as Basis for Learning, MUGI) observation checklist as a measurement instrument for gross motor skills and to introduce the MUGI model for motor training as an alternative to improve children's motor skills in practice. Methods. Motor skills observations were carried out on all pupils in the first three school years at a state-funded primary school in Sweden (n = 251). Pupils in an intervention group (n = 152) had one lesson of physical activity and motor training every school day and also, when needed, an extra motor training lesson per week. The MUGI observation checklist, which includes 16 items that measure static and dynamic balance, bilateral coordination, and eye-hand coordination, was validated and tested for reliability. Results. After one year of intervention, the pupils in the intervention group had significantly better (p < 0.05) motor skills than pupils in the control group. Results of factor analysis, reliability tests, and evaluator agreement show that the MUGI observation checklist fulfills requirements at an acceptable level of validity and reliability. The MUGI model was found to function well in practice for improving children's motor skills. Conclusions. The MUGI observation checklist could be an alternative to current measurement instruments in studies of children's gross motor skills, and also function as planning documentation for individual intervention programs. The MUGI model could be used to improve children's motor skills in practice.