<p>In-plane shear strength plays a critical structural role of carbon fiber reinforced polymer (CFRP) laminates. However, the lack of reinforcement in the through-thickness direction (i.e., z-direction) makes a CFRP laminate susceptible to catastrophic failure when subjected to shear stresses between the angle plies. Previous studies reported that using interleaves of unaligned nano particles between the plies had negligible in-plane shear strength improvements. This study introduces a novel approach for improving in-plane shear properties through the use of carbon nanofibers (CNFs) z-threaded CFRP (ZT-CFRP), which utilizes electrically aligned CNFs as nano-reinforcement to thread through the gaps between carbon fibers along the z-direction and forms a mechanically interlocked multiscaled fiber reinforcement network. In-plane shear tests were conducted on [45/-45]_4s laminates of control CFRP, 0.5wt% unaligned CNF-modified CFRP, and ZT-CFRP samples containing 0.5 wt% and 1.0 wt% CNFs dispersed in the epoxy matrix. Compared with control CFRP, the 0.5wt% and 1.0wt% ZT-CFRP samples showed significant 25.6% and 22.2% increases in the in-plane shear strength and the modulus of toughness, respectively. While the control CFRP samples exhibited both intralaminar and interlaminar delamination failure modes, with damage spreading across the plies, the ZT-CFRP samples demonstrated a more localized failure confined to the interlaminar region, which indicated a significantly reinforced intralaminar region. In contrast, unaligned CNF-modified CFRPs were weaker and more brittle than both control CFRP and ZT-CFRPs in the testing values and microscopy. This study proved the importance of using advanced nanostructure arrangement when adding nanofibers inside CFRP laminates to improve the in-plane shear performance.</p>