•Evaluation of surface roughness effect on multiaxial fatigue behavior of AM specimens.•Effect of surface machining and HIPing on multiaxial fatigue performance of AM specimens.•Fracture-mechanics based modeling of surface roughness effect on uniaxial and multiaxial fatigue performance. Additive manufacturing (AM) has become a very popular topic recently due to its many advantages including short build cycles, convenience of customization, and most importantly the ability to build components with complex geometry. However, the surface condition of additive manufactured components is not always satisfactory, particularly with respect to fatigue performance. This is because the as-built surface tends to be rough and post surface treatments or processes such as machining and polishing may not be applicable to all AM parts. On the other hand, since many components are under cyclic loading consisting of normal and shear stresses, multiaxial fatigue behavior is one of the most important aspects to evaluate. This paper evaluates the surface roughness effect on fatigue behaviors of Ti-6Al-4V alloy samples additively manufactured by laser-based powder bed fusion method (L-PBF). Fully reversed axial, torsional, and combined axial-torsion fatigue tests were conducted on specimens with different surface conditions and with different post heat treatments (annealed and HIP). Fatigue life predictions were made using linear elastic fracture mechanic with satisfactory results, as compared to experimental results.