Large-scale carbon nanotube (CNT) films fabricated by floating catalyst chemical vapor deposition (FCCVD) are promising reinforcement materials for high performance composites. However, little research has been reported on the four independent in-plane engineering constants of FCCVD CNT films, which are essential for understanding and prediction of mechanical behavior of FCCVD CNT film-based structures. The tensile strength in CNT-oriented direction is 127 MPa and the tensile modulus in oriented and transverse directions are 3.0 and 0.3 GPa, respectively. These mechanical properties are mainly attributed to the as-grown CNT-to-CNT contacts in the films. The Poisson's ratio in the oriented direction at 5 % strain is 0.75. A negative Poisson's ratio of -0.99 is observed at 0.1 % strain in CNT-oriented direction. The in-plane shear modulus is 0.57 GPa, which is derived from the coordinate transformation between the on-axis and 45A degrees off-axis compliance matrices. The in-situ scanning electron microscopy is adopted to observe the microstructure at different tensile strains. During the tensile testing, the reorientation of CNT bundles in CNT film is evaluated by numerical image processing and Raman spectroscopy.