The effect of X-ray irradiation on the line-driven stellar wind in X-ray binary systems is studied. The product of the X-ray luminosityL x and the densityn in the wind is a measure to distinguish between the optically thin and thick wind regimes. For an X-ray source of the radiation temperature of 10 keV, the critical value(L x n) c , is of the order of 1037 erg s-1 cm-3; hence, most of wind-fed X-ray sources lie in the optically thick wind regime because ofL x n>(L x n) c . Then the wind structure is determined not only by the parameter ζ=(L x /nR 2),R the distance from the X-ray source, but also by the optical thickness due to helium. The formation of fully ionized helium region depends onL x andn in a way different from that of the Strömgren sphere. In such an optically thick wind, the region behind the He II ionization boundary is little affected by X-ray irradiation and the trace elements remain to be responsible for wind acceleration. Thus, its location is important for the structure of the wind and the interpretation of various phenomena in objects such as wind-fed X-ray pulsars.