Silver (Ag) nanoparticles fabricated by chemical reduction process were grafted onto the surface of carbon nanotubes (CNTs) to prepare hybrid nanocomposites. The Ag/CNT hybrid nanomaterials were characterized using transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The Ag/CNT hybrid nanomaterials were then loaded in paraffin wax, and pressed into toroidal shape with thickness of 1 mm to evaluate their complex permittivity and complex permeability by scattering parameters measurement method in reflection mode using vector network analyzer. The reflection loss of the samples was calculated according to the transmission line theory using their measured complex permittivity and permeability. The minimum reflection loss of the Ag/CNT hybrid nanocomposite sample with a thickness of 1 mm reached 21.9 dB (over 99 % absorption) at 12.9 GHz, and also exhibited a wide response bandwidth where the frequency bandwidth of the reflection loss of less than -10 dB (over 90 % absorption) was from 11.7 to 14.0 GHz. The Ag/CNT hybrid nanocomposite with thickness of 6 mm showed a minimum reflection loss of similar to-32.1 dB (over 99.9 % absorption) at 3.0 GHz and was the best absorber when compared with the other samples of different thickness. The reflection loss shifted to lower frequency as the thickness of the samples increased. The capability to modulate the absorption band of these samples to suit various applications in different frequency bands simply by manipulating their thickness indicates that these hybrid nanocomposites could be a promising microwave absorber.