The ever growing ULSI device integration now requires ultra thin films as low as several nano-meter level. The so-called layer by layer growth is the ideal for making thin films for these requirements but island growth often occurred in metal CVD to deposit Al, W and Cu thin films on barrier layers, mostly composed of TiN or Ta/TaN films. We have made in-situ monitoring of initial nucleation behavior using laser light reflectivity measurement. This technique enables us to monitor the incubation period during which no nuclei formation is observed. The surface adsorbates during the incubation period was monitored by XPS and we found that the reactivity of initial surface and the film precursor is determining the incubation period.