In this work the behaviour of physically short mode I cracks subjectedto constant and variable amplitude loading conditions was investigatedA dynamic elastic-plastic two-dimensional finite element programpreviously developed for the simulation of crack tip deformation in fatiguewas extended to model the transient effect in crack tip advance as a result of the application of a single overload. Twenty three cycle patterns were analyzed to include the parameters responsible for the transient behaviour of physically short cracks.The extents of the plastically deformed zone for the base loadpreceding, during and following the application of a single overload wererecorded. The development of the engineering shear strain was alsodetected.The effects of crack length, the maximum stress of the single overloadto the material yield strength ratio, crOL / cry , in addition to the base load, BL,and the overload ratio, OLR, crOL /crBL , on the crack tip deformation, CTD,behaviour of physically short cracks, PSC, were studied. A single overloadcauses an acceleration if crOL > cry regardless the value of OLR. Otherwise,the retardation caused by a single overload increases with increasing OLRup to a certain value depending on the values of crOL / cry and crBL / cry ,and