Nanoscale materials with size smaller than the characteristic domain size could simplify the domain structure and uncover the intrinsic properties in detail. Herein, a ultrafast open space calcination pathway is first put forward to synthesize high-quality single-domain VO(2)(M) nanocrystals and an in situ variable-temperature IR spectroscopy is first proposed to identify the size-dependent MIT behaviors in VO(2)(M) below single-domain size. The variable-temperature IR spectroscopy clearly reveals that these single-domain VO(2)(M) nanocrystals exhibit new size-dependent MIT behaviors, while the IR analysis further suggests that the size-related defect density and scattering efficiency could be used to account for their novel size-dependent MIT behaviors. This new characterization strategy of in situ variable-temperature IR spectroscopy holds great promise for extending to other systems to gain valuable insight into their intrinsic phase transition behaviors. Also, this ultrafast open space calcination pathway sets forth a new avenue in fabricating high-quality functional nanocrystals and paves the way for constructing intelligent nanodevices in the near future.