One-step synthesis of a carbon nano sheets-carbon nanotubes composite by plasma enhanced chemical vapor deposition and its field emission properties are investigated. We obtain the carbon nano sheets-carbon nanotube composite on the Co thin film with 20 nm Ti interlayer. We gain carbon nano sheets only on the Co thin films without Ti interlayer in the same growth conditions. The carbon nano sheets are distributed on the side wall and the top of carbon nanotubes. The Ti interlayer hinders the diffusion of Co into the silicon substrate and improves the catalytic capability of Co, thus it will help the growth of carbon nanotubes. When the thickness of Co film is 11 nm, the carbon nanotubes are vertically aligned on the flat Co film surface. Most of the carbon nano sheets are distributed on the top of the carbon nanotubes under this condition, which increases the number of emitters and enhances the field emission properties of the composites.