The Modular turbo compressor shaft (4) comprise a tubular bearing portion (5) having a first axial end portion and a second axial end portion; an impeller portion (6) arranged at the first axial end portion of the tubular bearing portion (5); and a driving portion (7) arranged at the second axial end portion of the tubular bearing portion (5). The tubular bearing portion (5) is made of a hard material, and the impeller portion (6) and/or the driving portion (7) are made of relatively soft material compared to the hard material of the tubular bearing portion (5). The impeller portion (6) and/or the driving portion (7) are at least partially extending into the tubular bearing portion (5) and are firmly connected to the tubular bearing portion (5). The tubular bearing portion (5), the impeller portion (6) and the driving portion (7) are configured such that radial deformations, occurring during assembly of the impeller portion (6) and/or the driving portion (7) to the tubular bearing portion (5), are substantially limited to the driving portion (7) and/or the impeller portion (6).