Study of neurophysiologic mechanisms of arithmetic task solution is one of the most challenging problems of contemporary science. Task solving is as a complex structure process (van Harskamp et al., 2001) realized through a number of stages, which depend on a complexity of example (Serrien et al., 2004). The absence of the control of the indicators of the task solving may be the cause of the reported differences in the cortex activation pattern at the similar task solving (Zhou et al, 2007). The research was aimed at the study of mechanisms of activation and interactions of the brain cortex structures at different stages of two-digit addition and multiplication arithmetic task solving. The study involved 28 healthy volunteers. In Test1 40% of the tasks were represented only once, 30% of the tasks were repeated twice, and 30% of the tasks had the same operands. In Test2 the subjects received the following instruction in order to define the stages of the task solving: the “mouse” manipulator button was to be pressed each time after the obtainment of an intermediate result during the solving of each task. The stimuli were presented using the “Audio-visual Slider” program. The electroencephalogram (EEG) was recorded by 21 electrodes using electroencephalograph-analyzer Encephalan-131-03 (Medicom-MTD, Russia). The values of the solution time (ST), the number of operation, spectral power of EEG (SP EEG) and the coherence values (COH) were analyzed using the MATLAB. The confidence of differences was assessed by MANOVA-analysis. In Test1 no differences were revealed in the values of addition tasks. The differences were observed in multiplication only, because the singly presented examples were characterized by the longest ST values compared to the repeated examples and examples with similar operands. In Test2 when solving the addition tasks the algorithm might have included 1 – 4 operations. The ST values linearly increased alongside with increase in the number of operations from 1 to 4. Solving of multiplication tasks might have included 1 – 5 operations. When solving the multiplication tasks, the dome-shaped dependence of the ST was observed with increase in operation number from 1 to 5. Solving of addition tasks induced formation of EEG foci in delta-theta-band were formed in fronto-central and parietal regions of the left hemisphere and in parietal-temporal and frontal areas of right one. In multiplication tasks, delta-theta foci were formed in frontal, central and parietal regions of the left hemisphere as well as in temporal areas of the right one. When solving addition tasks, the coherence asymmetry was increased in left hemisphere between frontal, parietal and temporal areas in delta-theta-band as well as between frontal and parietal regions in alpha-band.The method, which we used in order to control the composition and duration of mental operations at the task solving, confirmed the suggestion that solving of addition and multiplication tasks required different skills, the realization of which was followed by different EEG-pattern and rearrangement of coherent connections.