The aim of the study was to assess whether the body build has an impact on hand grip strength, muscle endurance and time reaction in children. The material of the research were 376 children from 6.78 to 11.82 years old in 2018. Body weight, height, waist, hip and mid-upper-arm circumferences, triceps and subscapular folds were measured. The BMI, body fat and AHtR (arm-circumference-to-height ratio) were calculated. The hand grip strength, simple reaction time (Quickstick) and strength of the abdominal muscles (endurance) were tested. Two-way ANOVA and correlation analyses with significance level p < 0.05 were used. Children with normal BMI values are characterized by better simple reaction time than their obese peers, while overweight and obese children estimated on the basis of BMI, AHtR and body fat perform were better in hand grip strength. In the case of strength of the abdominal muscles, there were no differences in children with different body build. The normal build and body weight promotes the proper development of children, which also is reflected in the results of motor performance and fitness tests.