The effect of a low‐temperature‐grown gallium antimonide (LT‐GaSb) layer on the formation of high‐density GaSb islands on a Si(100) substrate was studied using ultrahigh‐vacuum scanning tunneling microscopy (UHV‐STM) and atomic force microscopy. By using an LT‐GaSb layer in the initial growth stage (new growth method), high‐density and small GaSb islands were formed at 500 °C regardless of the type of the reconstructed surface. The density and size of the GaSb islands with the LT‐GaSb layer were estimated to be ∼1.0 × 10¹¹ cm⁻² and ∼20 nm, respectively. The STM results showed that the LT‐GaSb layer was composed of an Sb‐rich amorphous GaSb layer. With a rapid increase in temperature under the Sb flux, the LT‐GaSb layer immediately changed into GaSb nuclei with a high density (2.1 × 10¹¹ cm⁻²) and small size (11 nm). It is suggested that these high‐density and small GaSb islands grow from the GaSb nuclei, which act as crystal seeds.