Composite metal decking has become one of the prevalent forms of Fast Track Construction for multi-story steel buildings. The in-plane stiffness of composite metal decks however is of considerable importance as they typically act as a diaphragm in lateral-load- resisting systems and thus attract a large in-plane shear. The diaphragm action or the in-plane shear capacity of composite metal decks remained a concern for structural engineers and designers. The present work aims to investigate the in-plane shear behavior of fiber reinforced composite metal decks and compare it to that reinforced with welded wire mesh. A comprehensive experimental program involving testing large scale slabs was conducted. Steel decking of trapezoidal profile with a sheet thickness of 0.9 mm was used to construct the composite slabs. Conventional WWF mesh reinforcement and synthetic macro fibers at different reinforcing rates were considered in the test program. The slabs were tested under monotonically increasing in-plane shear force until failure. Load-deflection responses were plotted and cracking pattern and sequence were recorded. The test results show that fibers provide comparable deck behavior as that with steel mesh. Both imparted similar ultimate in-plane shear capacity and enhanced the ductility and the post peak behavior of the composite metal decks relative to the control slab. The results demonstrate the viability of synthetic macro fibers to replace the steel mesh in this type of composite flooring systems.