The self-assembly of a newly synthesized, amphiphilic comb block copolymer (CBCP) at the air-water interface was systematically explored using the Langmuir-Blodgett (LB) technique. The CBCP had an ultra-high molecular weight (Mw = 510 × 103 g mol-1) with polystyrene arms grafted along one block of long hydrophilic backbone. At the air-water interface, the CBCP molecules spontaneously assembled into ribbon-like structures and cellular patterns at zero surface pressure when a volatile solvent (i.e., chloroform) and a less volatile solvent (i.e., toluene) were used, respectively. This spontaneous self-assembly behavior of the CBCP was induced by the dewetting process. The mechanism for the morphological change as a function of surface pressure was scrutinized and further confirmed by compression-expansion cycle and solvent vapor annealing studies. To the best of our knowledge, this is the first study of self-assembly of ultra-high-molecular- weight, amphiphilic CBCPs at the air-water interface. As such, it provides insight into the design of controllable pattern formation using amphiphilic copolymers.