In a dual-frequency capacitively coupled Ar plasma, the secondary electron emission (SSE) coefficients were estimated using 2D fluid model and experiments. The electron density was measured in the plasma chamber with the upper and lower electrodes of Si. The electron density was calculated by changing the SEE coefficient. The SEE coefficient of the Si electrode was estimated to be 0.06 so that the electron density of the 2D fluid model was close to that of the experimental result. Next, the electron density was measured in the chamber of the lower electrode covered with SiO2 and polyimide and the SEE coefficients were estimated by comparing experiments and 2D fluid model. The SEE coefficients of the SiO2 and the polyimide were estimated to be 0.24 and 0.22, respectively. Moreover, The SEE coefficients of SiO2 was almost the same in the range between 300 and 500 W of the lower electrode power.