  Development of new, high quality functional materials has been at the
forefront of condensed matter research. The recent advent of two-dimensional
black phosphorus has greatly enriched the material base of two-dimensional
electron systems. Significant progress has been made to achieve high mobility
black phosphorus two-dimensional electron gas (2DEG) since the development of
the first black phosphorus field-effect transistors (FETs)$^{1-4}$. Here, we
reach a milestone in developing high quality black phosphorus 2DEG - the
observation of integer quantum Hall (QH) effect. We achieve high quality by
embedding the black phosphorus 2DEG in a van der Waals heterostructure close to
a graphite back gate; the graphite gate screens the impurity potential in the
2DEG, and brings the carrier Hall mobility up to 6000 $cm^{2}V^{-1}s^{-1}$. The
exceptional mobility enabled us, for the first time, to observe QH effect, and
to gain important information on the energetics of the spin-split Landau levels
in black phosphorus. Our results set the stage for further study on quantum
transport and device application in the ultrahigh mobility regime.
