The carbon nanotube-metal nanoparticle complex has attracted a lot of research interest because of their potential applications in catalysis and gas sensing. Here we introduce a novel electrochemical method to realize in-situ decoration of single-walled carbon nanotube field effect transistors (CNT-FET) with metal nanoparticles using a sacrificial electrode. In this process, metal atoms are first ionized into an electrolyte solution by applying a potential difference between the sacrificial electrode and the grounded source/drain electrodes connecting the nanotube of the CNT-FET. The positive metal ions migrate under the influence of the electric field, and deposit on the grounded nanotube as metal nanoparticles. This method provides for better control over the quantity and size of the deposited nanoparticles compared to other decoration methods. We demonstrate successful deposition of Au and Ag nanoparticles on carbon nanotube field effect devices, with the quantity and size of the nanoparticles varying as a function of the applied potential. We show that the metal nanoparticle size can vary from 10 nm to over 300 nm, and the spatial distribution can change from very scarce decoration to a near continuous coating. Such metal nanoparticles have potential applications in chemical sensors, as they interact with gas molecules and generate an electrical signal in the nanotube, which can be detected. They can also be explored as biological anchoring sites for bio-functionalization of the nanotube, which is critical to developing highly sensitive and selective bio-sensors.