The fatigue hfe of a quenched and tempered AISI 4340 steel has been evaluated in three different conditions: (a) uncoated, (b) coated with an electroless Ni-P (EN) deposit of a P content of approximately 12-14wt.%, as-deposited and (c) as-deposited, followed by a two-step post-heat treatment (PHT): 473 K for 1 h plus 673 K for 1 h, The results indicate that plating the base steel with this kind of deposit leads to a significant reduction of the fatigue life of the material, particularly if the deposit is subjected to a subsequent PHT. Such a reduction has been quantified by determining the Basquin parameters from the fatigue life curves obtained for the uncoated, coated, coated and PHT substrates. It has been shown that the fatigue life of the base steel can be reduced by 78% in the as-deposited condition and a 92% after a subsequent PHT. The microscopic observation of the fracture surfaces of the samples indicate that the fatigue process is initiated at the surface of the deposit and, subsequently, transferred to the substrate, with the assistance of the metallic bonding established at the deposit-substrate interface. This belief is supported by the observation of some continuity of the fracture features between the coating and the substrate under low alternating stresses. In the present study, the bonding between the EN deposit and the base steel was observed to be rather poor. Extensive secondary cracking along the coating-substrate interface after fatigue testing as well as the complete separation of the deposit from the substrate during tensile testing support this view. Such a behavior is believed to be related to the significant difference that exists between the elastic and plastic properties of the EN deposit and the base steel, Nonetheless, the slight degree of metallic bonding that remains after the first stage of fatigue testing seems to be enough to allow the passage of the fatigue cracks, prior nucleated in the deposit, into the substrate. It is therefore concluded that, in the present case, the EN deposit acts as a surface crack source or surface notch which decreases the fatigue life of the coated material by reducing the crack nucleation stage.