The position of the dentition is considered to be determined by a combination of forces exerted by the perioral muscles and tongue. We hypothesized that mandibular anterior crowding may be related to abnormalities in the development of oral function. To determine the relationship between oral function and mandibular anterior crowding in early mixed dentition. A total of 61 children (30 boys, 31 girls) with early mixed dentition were included in this study. Height, body weight, maximum occlusal force, lip-closing strength, and maximum tongue pressure were measured in all participants, and their dental casts and lateral cephalograms were evaluated. Little's irregularity index (LII), evaluated by dental casts, was used as an indicator of mandibular anterior crowding. Maximum occlusal force and lip-closing strength were not significantly correlated with the LII, although they were positively correlated with maximum tongue pressure, mandibular intercanine perimeter distance, and upper central incisor (U1)-to-NL angle (p < .05). Maximum tongue pressure was negatively correlated with LII (p < .05). Maximum tongue pressure and LII were significantly positively correlated with the mandibular intercanine perimeter distance and U1/NL angle, and negatively correlated with the interincisal angle (p < .05 for all). Crowding of the mandibular anterior teeth was directly correlated with tongue pressure function and indirectly correlated with maximum occlusal force and lip-closing strength.