Clathrate hydrates have been paid attention in tenns of industrial application as high density gas storage material, sea water desalination and so on. However, Gas hydrates tend to f 01m hydrate film between gas and water. As a result, guest and host phases were separated and the fmther fonnation of hydrate crystals was limited without string the sample. This study presents growth of tetrahydrofuran (THF) clathrate hydrates in guest/host concentration bounda1y between THF and water, to clarify the growth mechanism of hydrate c1ystals in typical growth situation of two phase boundaiy. Our previous study [1] first showed growth mode transition from the lateral growth of hydrate film in the guest host concentration boundary to continuous nucleation around the initial nucleation area of the hydrate film. The successive nucleation resulted in expansion of the polycrystalline aggregates into both phases. In the present study, interferometric observation of the growth process was caITied out as well as optical microscope observation, because the diffusion of THD and water is one of rate-dete1mining processes. First of all, the pure water was injected into a vertically aiTanged growth cell kept at a temperature(= -2.0~3.0 °C) below the equilibrium temperature (= 4.4 °C) of the THF hydrate in the stoichiometric THF water solution. Then, THF was cai·efully injected on the surface of water from above. 0   Llwt-0/o Contacting a chilled wire to the concentration boundaiy layer between water and THF, hydrate c1ystals were nucleated. The growth process was in-situ observed using an Owl% optical microscope and an interferometer to show the growth speed and thickness of the hydrate film, expansion of polycrystalline aggregates, and concentration field of THF in water. In addition, the initial thickness of THF/water concentration boundaiy layer was observed as one of pai·aineters affecting the hydrate film thickness, lateral growth rate, and the induction time for the onset of successive nucleation to fonn polyc1ystalline aggregates. The thickness of the concentration boundai·y layer was defined as the region where the THF concentration is between 1.1 ~97 wt<>/o, as shown in Fig. 1. Note that the initial thickness of the concentration boundaiy layer depends on the experimental mns, because the THF diffuse into water phase during the pouring ofTHF on water. This is the unknown factor in our previous study [8]. The results showed that the hydrate film thickness is proportional to the initial thickness of the concentration boundai·y layer, L. Lateral growth rate of the hydrate film was affected by the value of L and the sainple temperature. Fmthe1more, the expansion rates of polyc1ystalline agglomerate of hydrates were affected not only by the temperature but by the hydrate film thickness propo1tional to L. As the film thickness is large, the expansion rate was low. This suggests that the mass transpo1t of THF and water through the hydrate film was rate detennining process of the expansion of polycrystalline agglomerate. Then, observing the areas STIIF and Sw of the agglomerates in both water and THF phases, it was found that the ratio of SmF to Sw was constant during each nm. The ratio was ai·ound 0.15~0.37. Slower expansion rate of THF phase can be explained by the phase diagram of hydrates in THF-water solution. Details will be mentioned in the conference.