Laser-induced nanomodification on hydrogen (H)-passivated Si(100) surfaces has been carried out underneath the tip of a scanning tunneling microscope (STM) in ambient air. The created features were characterized using STM, atomic-force microscopy (AFM), and Auger electron spectroscopy (AES. The features appeared as depressed regions in STM images, while they appeared as protruded regions in AFM images. Oxygen was detected in a modified 2x2 mu m(2) square region by AES, while no oxygen was detected elsewhere on the same sample surface. The experiment results and mechanism are discussed. Nano-oxide patterns, such as lines and dots, have been created. Dependence of oxide apparent depth on laser intensity, laser pulse numbers, tunneling current during modification, and laser incidence angle has been investigated.