The in-line hydrocarbon(HC) adsorber is one type of passive after-treatment technology which can reduce the cold start hydrocarbon from automobile engine exhaust gas. However, major technical issues to be resolved for practical use of this system are 1)the temperature difference between HC desorption from the adsorbent and activation of the catalyst to burn-off the desorbed HCs and 2)the ability to adsorb a wide range of HC molecule sizes in the cold exhaust gas.Recent zeolite development has resulted in an unique adsorbent capable of adsorbing small HC molecules as well as large to some degree, even under aqueous conditions. This adsorbent also features a relatively high HC desorption temperature. As a result, an In-line HC adsorber system has been developed, which can trap more unburned HCs during the engine cold start phase and burn-off the desorbed HCs more efficiently in the desorption phase by using this unique adsorbent. This HC adsorber system demonstrated a drastic reduction in FTP NMHC emission.This paper describes our evaluation results of the improved HC adsorber including a discussion on the thermal durability of the adsorbent.This In-line HC adsorber system is one of the viable tools to address future tighter regulations as a “stand-alone” system or as a component within other after-treatment systems to increase their effectiveness.