Objective: To examine the effect of age, sex and obesity on Fundamental Motor Skills (FMS) in 4 to 6 years-old children. Methodology: A total of 400 preschool children (200 boys and 200 girls) between the ages of 4 to 6 years old participated in this research. Subjects were selected through multi-stage cluster random sampling. Fundamental motor skills (FMS) were assessed with using the OSU-SIGMA scale. Body mass index (BMI) was directly measured from height(m)2/weight(kg) for each child and based on CDC growth charts, normal weight, overweight and obesity were defined. Results: The results showed that age and sex variables were a significant effect on walking and running skills, but BMI was not significant (P>0.05). Also, these variables had a significant effect on jumping, skipping, hopping and ladder climbing. In both ages, boys in jumping and ladder climbing skills were better than girls, but the girls were better in skipping and hopping skills (P<0.05). Moreover, the results showed that age and BMI variables have a significant effect on stair climbing skill, but sex was not significant (P>0.05). For object control skills, the results showed that age and sex variables were a significant effect on catching and throwing skills, but BMI was not significant (P>0.05). Finally, the age, sex and BMI variables were a significant effect on kicking and sticking skills. Conclusion: This research demonstrated that boys performed better than girls, and both overweight and obese children have lower performance than normal children.