Shrinkage porosities are the main cause of fatigue crack initiation in cast steel components subjected to cyclic loads. Advanced foundry processes can eliminate large shrinkage pores by optimizing the component geometry, the mold and the feeding system. However, micro-shrinkage pores always remain present and their size is generally in the order of several hundreds of micrometers. The impact of these pores on the fatigue resistance has been extensively documented for cast aluminum alloys, but there is a lack of quantitative studies concerning cast steels. The study aims at demonstrating how micro-shrinkage pores affect the fatigue resistance of the G20Mn5 cast steel under high-cycle fatigue (HCF) conditions for uniaxial and torsional loads. It investigates the size, position, geometry, and distribution of micro-shrinkage pore populations and examines three specimen configurations of cast G20Mn5 steel with varying micro-shrinkage pore sizes while maintaining a similar microstructure, determined by a normalization heat treatment after casting. An almost porosity-free material obtained using the HIP process is also studied as a reference material. HCF tests (censured at Nmax=107cycles) in tension-compression (R=-1) and torsional (R=-1) loading conditions were performed on cylindrical specimens. The fatigue failure surfaces of all the failed specimens were observed using scanning electron microscopy (SEM) to characterize the critical pore at the crack initiation site. The size, distance to the surface, and circularity were measured. 3D tomography tests were conducted on certain fatigue specimens before and after fatigue testing to assess the impact of the real micro-shrinkage distribution on the fatigue resistance. Additionally, fatigue specimens containing artificial defect will be loaded, in combination with triggered optical observations to measure the on-surface crack propagation rate. The preliminary results indicates that for the configurations containing pores, fatigue cracks initiate at surface pores. While for the “pore-free” configuration, cracks typically initiate at inter-metallic inclusions. The study uses Kitagawa-Takahashi diagrams to examine how pore size affects the fatigue strength. Differences in the fatigue strength, between the uniaxial and torsional loading conditions are observed, with a less sensitivity to the pore size for pure shear loading mode. A statistical simulation based on X-ray tomography data and using a Kitagawa type model illustrates how the distribution of the micro- shrinkage pores affect the fatigue resistance. This PhD project is a French CIFRE collaboration between Safe Metal and the LAMPA laboratory of the Arts and Métiers Institute of Technology.