The application of high velocity oxygen fuel (HVOF) coatings has gained increasing acceptance in the aerospace Industry. It has the potential to replace hard chromium coatings in a number of applications. This work was focused on ascertaining the limitations of HVOF coatings applied to ultra high strength steels and components experiencing high loading stresses. A group of tubular axial fatigue specimens with 2.3 in. diameter and 5.0 in. long gage section were coated with a WC-Co composite coating (Suizer Metco Diamalloy 2005) via the HVOF process. HVOF coating thickness was varied from -0.006 in. to -0. 0l2 in. and substrate material used was 4340 steel to HyTuf. The fatigue specimens were subjected to 20 cycle test segments starting at 150 ksi or 160 ksi for stress ratios of-I and 4).33, respectively. Stepped stress testing was continued in 10 ksi increments until coating failure was observed in the form of cracking and/or spalling. Coating failure was observed to be a function of coating thickness. As coating thickness increased, coating failure occurred at progressively lower stress levels.