The fluorescence imaging in the second near-infrared window (NIR-II, 1000-1700 nm) has emerged as a new method for in vivo imaging and attracted considerable attention in the past decade. Owing to the suppressed photon scattering and diminished autofluorescence, in vivo fluorescence imaging in NIR-II window can afford deep tissue penetration depth with high clarity. Inorganic nanoparticle-based fluorescent probes in the NIR-II window have greatly prospered the field into a development stage because of their superior traits, including adjustable emission covering the whole NIR-II window and abundant surface functional groups that facilitate chemical modification and bioconjugation, etc. In this Feature, we introduce the unique imaging performance of the NIR-II optical window and highlight the latest development of noninvasive biological fluorescent imaging in NIR-II window using inorganic nanoparticle-based probes. A perspective on the challenge and future direction of inorganic nanoparticle-based NIR-II probes is also discussed.