We proposed a precision mathematical model for quantitative evaluation of micro-dynamics of cultured cells measured with ECIS (Electrical Cell-substrate Impedance Sensing) based on equivalent electrical circuit that could separately estimate cell-to-cell distance and cell-to-substrate distance. The model was composed of three parts, the cell impedance, culture medium electrolyte impedance between cells in perpendicular direction to electrode, and polarization impedance of the electrode. The cell impedance, mainly cell membrane impedance is formed by the equation of Cole-Cole dispersion. We measured the frequency characteristics 25 Hz to 60 kHz of impedance of BAEC (bovine aortic endothelial cell), in full confluent condition of each cell with ECIS. The mathematical models of these impedances could be fit well especially from 1 kHz to 10 kHz, which were most interesting frequency range for micro-dynamic analysis in ECIS method. Based on this precision model, we could propose the evaluation method of the cell-to-cell distance A and the cell-to-substrate distance h. Namely, in order that the micro-motions of A and h can be analyzed easily by vector impedance change based on the mathematical model, we introduced new parameters SA and Sh. If vector impedance value is obtained in each condition, parameter values correspond to the vector impedance variation can be instantly determined and evaluations of micro-motion of the cells can be performed. If A decreases, SA decreases because culture medium resistance Rsol increases and if h decreases, Sh decreases because polarization impedance Z0 increases. We investigated the effect of X-ray radiation exposure from 1 Gy to 100 Gy to the cultured cell BAEC by ECIS. The impedance change could be confirmed from just after X-ray exposure. The X-ray stimulation of 100 Gy resulted in the large scale of increase in the cell-to-cell distance A and the slight decrease in the cell-to-electrode distances h.