  From the extreme position of disk motion, we infer the neutron star (NS)
surface magnetic field strength B_0 of Z-source GX 17+2 and Cyg X-2. The
inferred B_0 of GX 17+2 and Cyg X-2 are ~(1-5)X10^8 G and ~(1--3)X10^8 G},
respectively, which are not inferior to that of millisecond X-ray pulsars or
atoll sources. It is likely that the NS magnetic axis of Z sources is parallel
to the axis of rotation, which could result in the lack of pulsations in these
sources.
