Understanding the formation process in the controlled synthesis of nanocrystals will lead to the effective manipulation of the morphologies and properties of nanomaterials. Here, in-situ UV-vis and X-ray absorption spectroscopies are combined to monitor the tracks of the nucleation pathways in the solution synthesis of platinum nanocrystals. We find experimentally that the control over nucleation pathways through changing the strength of reductants can be efficiently used to manipulate the resultant nanocrystal shapes. The in-situ measurements show that two different nucleation events involving the formation of one-dimensional "Pt(n)Cl(x)" complexes from the polymerization of linear "Cl(3)Pt-PtCl(3)" dimers and spherical "Pt(n)(0)" clusters from the aggregation of Pt(0) atoms occur for the cases of weak and strong reductants; and the resultant morphologies are nanowires and nanospheres, respectively. This study provides a crucial insight into the correlation between the particle shapes and nucleation pathways of nanomaterials.