Formation process and thermal relaxation of three-dimensional islands like pyramids have been investigated by a temperature-variable scanning tunneling microscopy (STM). Isolated single islands on the Si(lll) (7 x 7) surface between 700 and 750 K have been produced using an STM. It is found that island is produced with a small distance between the tip and the sample substrate, and sample polarity does not affect island formation. Indices of main facets of the pyramid are (311), and small facets are (221). Two types of pyramids are produced. Islands with production probability of 75% are normal stacking at the interface between the island and the substrate, and called type N. For islands with production probability of 25%, which are in the twin relation of the type N islands, there is a stacking fault at the interface, and called type F. Decay rate of the type F island is larger than that of type N. During decomposition of type N, the facets of the pyramid are split into two parts. For the type F islands, the pyramids decay nearly layer-by-layer without splitting and step bunching.