The relationship between kink defects and Fermi level pinning on molecular beam epitaxially grown GaAs and InP(001)-(2x4) surfaces is studied in detail by scanning tunneling microscopy and x-ray photoelectron spectroscopy. In Si-doped GaAs, the kink density increased with doping as previously found. However, actual density depended very much on the reconstruction phases and experimental conditions. At high Si doping levels, Fermi level was strongly pinned below mid-gap, but the measured kink density was found not to be large enough to explain pinning by the previous kink-deep-acceptor model assuming that each kink forms a single discrete level. In Si-doped InP, the kink density remained constant with the increase of Si doping, although the Fermi level was pinned above mid-gap. The result cannot be explained by the kink-deep-acceptor model either.