Abstract Thermally cleaned InP surfaces in an arsenic flux have been observed using an ultra-high-vacuum scanning tunneling microscope (UHV-STM). The STM image of the reconstructed In-stabilized surface showed 1.6 nm periodic lines in the  direction. The width of these lines was about 0.8 nm, each line comprising two rows when the sample was heated at 510°C for about one minute in an arsenic flux. On the other hand, the lines were about 0.4 nm wide and comprised a single row when the sample was heated at a higher temperature for a longer time. This result suggests that the In-stabilized surface comprises one or two In-In dimers and missing dimers per 4 × 2 cell, and that the dimer density depends on the cleaning conditions. For samples annealed at 480°C in a vacuum of 10 −5 Pa for about ten minutes after cleaning, the surface image showed 4 × 2 In-stabilized domains and 2 × 4 As-stabilized domains. This structure is thought to be formed by a desorption of surface atoms during annealing.