The dependence of the absorption edge of wurtzite $Al_xGa_{1-x}N$ on alN mole fraction has been studied. The AlN mole fraction was varied from 0 to 1. The absorption coefficients at room temperature were determined by transmission and photothermal deflection spectroscopy. Photothermal deflection spectroscopy can be applied to determine the low absorbance values. From the results, the effective bandgaps of $Al_xGa_{1-x}N$ alloys were determined by choosing corresponding photon energies of the positions of the absorption coefficient of $6.3 imes10^4 extrm{cm}^{-1}$ at the absorption curves of the $Al_xGa_{1-x}N$ alloys. From the energy position of the absorption coefficient versus AlN mole fraction, a bowing parameter of 1.3eV was determined. The bowing parameter agreed quite well with the measured effective bandgaps of AlGaN alloys.