Extendibility of proximity x-ray lithography (PXL) relates to three technical issues. They are the minimum proximity gap usable for steppers, the maximum allowable gap by exposure methods, and overall blur effect. The minimum usable gap g(s) (mum) for steppers is presumed to be g(s)=W-1/2, where W (nm) is the design rule (half pitch). The maximum allowable gap, g is expressed as g = alphaW(2)/lambda, where lambda is the average wavelength and a is a coefficient, which depends on the exposure methods using 1X masks or 2X masks. In the article, we have verified the equation (g = alphaW(2)/lambda) by simulation for one- and two-dimensional patterns, and determined the a values for each exposure method. For the blur effect, we evaluated the sigma values using a simple model. As a result, we concluded that PXL can be used by 25 nm and most probably by 18 nm design rule by changing the exposure methods corresponding to the progress of the design rule generations.