We have studied the effective optical absorption power of Si solar cell with $SiO_2$-antireflection layer based on a mathematical modelling of AM(air mass)1 spectrum and Si refractive index in the wavelength range(0.4 $mu extrm{m}leq$λ$leq$$0.97mu extrm{m}$). The effective optical absorption power obtained from the theoretical calculation was 450 and 520 W/$m^2$ for the Si solar cells with $SiO_2$-antireflection layer of 500$AA$ and 1000$AA$, respectively. The optimum thickness of $SiO_2$-antireflection layer showing the minimum reflection loss was about 1000$AA$ in the computer simulation. Two kinds of Si solar cells named EBS(500$AA$) and EBS(l000$AA$) were fabricated to evaluate the effect of $SiO_2$-antireflection layer thickness on the optical absorption. The epitaxial base Si cell with $SiO_2$-antireflection layer of 1000$AA$ [EBS(l000$AA$)] showed the output power improvement of about 15% upon the EBS(500$AA$) cell due to larger absorption of effective optical power under illumination of AM1, 1 sun.