The effect of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) addition on the phase formation and the superconducting properties of GdBa2Cu3O7-delta phase has been studied. Therefore, composite superconductor samples of type (CNTs)x GdBa2Cu3O7-delta, 0.0 <= x <= 0.1 wt.% have been synthesized by a standard solid-state reaction technique. The samples have been characterized using X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results of XRD show an enhancement in the phase formation up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively. SEM and TEM reveal that CNTs form an electrical network resulting in well-connected superconducting grains. The electrical properties of the prepared samples have been examined by electric resistivity and IV measurements, and their results reinforce the XRD, SEM and TEM. Consequently, both T-c and J(c) improve as the addition percentage increases up to 0.06 wt.% and 0.08 wt.% for SWCNTs and MWCNTs, respectively.