In this study, contact potential difference (CPD) of the electrochemical co-deposited nanocrystalline nickel-cobalt (Ni-Co) alloy films were measured by a home-made Kelvin probe. We present not only the detailed sensing mechanism but also discuss two different methods of CPD determinations namely null and off null method. It is also compared with the work function (WF) measured by ultraviolet photoemission spectroscopy (UPS). In electrochemical co-deposited nanocrystalline Ni-Co films, the Co content decreases with increasing current density from 22.53% at 1 ampere per square decimeter (ASD), 20.8% at 5ASD to 15.26% at 10 ASD. It is particularly suitable for Kelvin probe calibration due to the composition difference. The diameter of Kelvin probe is about 4 mm, and electrodeposited by nickel. The WF of the probe head was evaluated via UPS, and the value is 4.1 eV. The traditional determination of CPDs is null method at low scan rate and noise sensitive while the off null method provides higher signal-to-noise ratio, effective measurement and also shorter process time. Both UPS and Kelvin probe method show the identical results that the WF increases with increasing current density because of low Co content with low WF. The results of null and off null measurements for CPDs are in good agreement with that by UPS.