Sibirskite and parasibirskite are relation of polymorph, and occur by the alteration products of takedite at Fuka, Okayama Prefecture. The synthesis of parasibirskite was reported, but the synthesis of the sibirskite has not been reported. To determine the formation condition of sibirskite and parasibirskite, they were synthesized from natural takedite. As a result, sibirskite and parasibirskite were formed when synthesizing it for 7 days at 1000 atmosphere and 300C and only sibirskite was formed with the synthesis for 14 days in this condition. In the synthesis from the reagent at 1000 atmosphere and 300C, sibirskite was not generated, but it was synthesized by addition of the seed crystal of sibirskite. Powder samples containing sibirskite and parasibirskite were synthesized under the hydrothermal condition. In any condition, the amount of sibirskite were increased during the run duration, and the amount of parasibirskite were decreased. These findings suggest that sibirskite is stable at these range of temperature and pressure, and parasibirskite is unstable though it formed at the first stage of generation.