Brain processing of syntactic material can be altered by bilingualism [1] (i.e. mastering of two languages simultaneously). According to Piaget J., cognitive development of children depends on their age and can be divided into 4 stages: sensorimotor, preoperational, concrete operational and formal operational. However, the effect of bilingualism on brain processing of sentence grammar and structure violations during these development stages in Mongolian bilinguals is not well known. The recent study aimed to investigate brain function in processing of syntactic information by analysing brain peak waves, such as LAN (left anterior negativity), N400 and P600 in Mongolian monolinguals (control group) and bilinguals.