We study the influence of surface defects on the structure of a Si(001) surface by statistical mechanical calculations based on an asymmetric dimer model. Recently, the local influence of surface defects has been identified by the scanning tunneling microscope (STM). The arrangement of images of dimers near the defect is well reproduced both at low and at high temperature in the calculated STM image. We also find that, in a particular configuration of a few defects, the influence of the defects is much wider than the simple sum of isolated defects at low temperatures. This gives a reason why a wide region of symmetric-appearing images was observed in the area with rather low defect density in a low-temperature STM image. © 1995 The American Physical Society.