  We theoretically demonstrate that the in-plane magnetization induced quantum
anomalous Hall effect (QAHE) can be realized in atomic crystal layers of
group-V elements with buckled honeycomb lattice. We first construct a general
tight-binding Hamiltonian with $sp^3$ orbitals via Slater-Koster two-center
approximation, and then numerically show that for weak and strong spin-orbit
couplings the systems harbor QAHEs with Chern numbers of $\mathcal{C}=\pm1$ and
$\pm2$ , respectively. For the $\mathcal{C}=\pm1$ phases, we find the critical
phase-transition magnetization from a trivial insulator to QAHE can become
extremely small by tuning the spin-orbit coupling strength. Although the
resulting band gap is small, it can be remarkably enhanced by orders via
tilting the magnetization slightly away from the in-plane orientation. For the
$\mathcal{C}=\pm2$ phases, we find that the band gap is large enough for the
room-temperature observation. Although the critical magnetization is relatively
large, it can be effectively decreased by applying a strain. All these suggest
that it is experimentally feasible to realize high-temperature QAHE from
in-plane magnetization in atomic crystal layers of group-V elements.
