Abstract: The limited spectrum resource has restricted continuing developments of many nations, while the classical channel capacity and spectrum utilization limit based on typical rectangular spectra partition, can not explain the highefficiency of some sinelike modulations already. Aiming at the sinelike signal characteristic of the ultra narrow band(UNB) modulations, a nonrectangular spectra partition model is proposed which has a monopolized ultra narrow “spectrum tree” plus shared broad “spectrum grasses”. Its technique reasonability is demonstrated in aspects of the full use of existing spectrum resources and the rational toleration of dense multicarrier information transmission scheme. The necessity is pointed out for parallelism derivation of the channel capacity expression and the unit spectrum utilization limit, based on the proposed nonrectangular spectrum partition model.