Formation of surface-induced nanopattern (SINPAT) in ultrathin diblock copolymer films is studied by scanning force microscopy and Monte Carlo simulation. The pattern is caused by strong adsorption of one of the two blocks forming a quasi-two-dimensional coil while the other block dewets this adsorption layer. Scanning force microscopy allowed to observe an order-disorder transition for a SINPAT film of polystyrene-block-poly(2-vinylpyridine) on mica when the length of the dewetting polystyrene block was varied. The experimental data are compared with the Monte Carlo simulations which demonstrate how the pattern formation depends on the degree of polymerization of the dewetting block and the unfavorable interaction potential between the different components.