Copper indium gallium diselenide (CIGS) is a promising photovoltaic material. Non-vacuum deposition of CIGS is a recommended strategy to produce cost effective solar cells. Amongst various non-vacuum deposition techniques, nanoparticle based deposition methods have gained major impetus due to their economic benefits, simplicity and flexibility to scale up. In the present work, CIGS nanoparticles are synthesized by a mechanochemical process and the effect of milling parameters (ball to powder ratio (BPR), milling speed (rpm) and milling time) on the structural, morphological and compositional properties have been studied. CIGS nanoparticles are synthesized with BPR of 15:1, 20:1 and 25:1 for different milling times ranging from 1 to 6 h and milling speeds from 200 to 400 rpm. The synthesized CIGS nanoparticles have been characterized using XRD, FESEM, HRTEM and EDAX analysis. XRD analysis showed the formation of chalcopyrite CIGS nanoparticles without any secondary phase within 2 h of milling time with a BPR of 25:1 at 400 rpm. The influence of milling parameters on morphology and agglomeration has been studied using FESEM. It is observed that the nanoparticles synthesized at higher BPR with shorter milling time, are less agglomerated. The compositional study performed by EDAX analysis showed that the synthesized CIGS nanoparticles are in good match with the desired stoichiometry of Cu(In,Ga)0.5Se2.