The goal of this research was to examine the hypothesis that weakness in brain holistic mechanism may explain the problem in grammar understanding in preschool children. 323 Russian-speaking children aged between 5 years 11 months and 6 years 10 months participated in the study. The children were assessed with the task “Comprehension of grammatical structures” from Luria's neuropsychological assessment battery that was adapted for preschool children. The experimental (grammatical weakness) and control group (grammatical strength) were formed. The Rey-Osterieth Complex Figure test was used to assess the holistic abilities in children. We have revealed that majority of children with poor grammar understanding had a piecemeal (immature) strategy when copying the Rey-Osterieth Complex Figure. The received result can be explained by the assumption that preschool children with poor grammar understanding have deficit of the specific brain mechanism responsible for holistic synthesis.