Clathrate hydrates provide porous structures for encaging gas molecules at a high volume ratio (v/v) and can be exploited to improve industrial gas storage and transportation. Direct clathrate hydrate formation from liquid water without mechanical agitation has yielded very low conversion rates due to the mass-transfer limitation between phases. In the present study, the heavy guest molecule of iodomethane was introduced to achieve both rapid clathrate hydrate formation and a high conversion rate. Iodomethane showed full enclathration in water cages in a few minutes and even accelerated the full conversion to a mixed hydrate (sII hydrates) with methane to 1.5 min by renewing liquid-iquid interfaces without mechanical agitation. The interfacial renewal at the growth front will contribute to developing a cost-effective method for natural gas storage and transportation in a clathrate hydrate form.