The surface state properties (such as surface state density, time constant and level position related to the bottom of InAlN conduction band) of Ni/Au/-InAlN Schottky barrier diodes with nearly lattice matched (InN=18%) and tensilely (15%) or compressively (21%) strained InAlN barrier layer were evaluated, by using current-voltage (I-V), frequency-dependent capacitance-voltage (C-V) measurements and atomic force microscope (AFM) images. It was found that, with increasing content of In the surface state density increased and the barrier height of the Schottky contacts decreased, respectively. The C-V curves shifted toward the positive bias values with reducing measured frequencies, which became more apparent with increasing In content. It may be due to the hole emission from the surface states of Ni/Au/-InAlN Schottcky contacts. Atomic force microscope (AFM) images indicated that the InAlN surface became rougher with increasing In content, which may be the main reason for the increased surface state densities.