The doping behavior of Si in AlGaAs with AlAs mole fraction from 0 to 1 was reported. Si-doped AlxGa1-xAs layers were grown by gas source molecular beam epitaxy with a constant Si cell temperature for all samples. The electrical properties and composition of the ternary alloys were characterized by Hall effect and X-ray diffraction,respectively. Results show that the electron concentration of Si-doped AlxGa1-xAs varying with Al mole fraction has a minimum value at x=0.38,which is the Γ-X direct-indirect band crossover of AlGaAs system. The Hall mobility decreases with the increasing of AlAs mole fraction till about x=0.4,hereafter it remains at a low value of mobility with small change rate.