The nucleus is the largest and stiffest organelle of eukaryotic cells, and as such, its mechanical properties are tightly related to various cell functions. Many efforts have been devoted to characterize the mechanical properties of nucleus, but the current techniques generally need physical contact of the cell and staining of the nucleus and thus cannot acquire the mechanical information directly. Brillouin microscope is an integration of a confocal microscope and a Brillouin spectrometer, which measures the spectral shift due to the spontaneous Brillouin light scattering, and from that the longitudinal modulus of the sample can be quantified. In this work, by combining the standard Brillouin microscope with the microfluidic technique, we developed a Brillouin flow cytometry that can quantify the mechanical properties of the intact cellular nucleus in a non-contact and label-free manner. As cell flows through a microfluidic channel, its mechanical property at different regions will be sampled by a sub-micron beam spot of the Brillouin microscope. The mechanical information of the nucleus from the cell population can then be identified and extracted via data post-processing, which is further confirmed by co-registering Brillouin data with fluorescence data from the same cell. Currently, the overall throughput of this technique is about 200 cells per hour, mainly relies on the acquisition speed of the spectrometer, which could be readily improved with available technology. We verified the capability of this all-optical technique by distinguishing the stiffness changes of the nucleus that are relevant to physiological and pathological phenomena.