We report an in situ observation of temperature-dependent phase transition in MnAs thin film by transmission electron microscopy (TEM) techniques. Following the identification of the crystallographic transition, from hexagonal α-MnAs to quasihexagonal β-MnAs, the orbital-to-spin moment ratio is measured and a breaking of the ferromagnetic order locally observed, thanks to the electron magnetic chiral dichroism (EMCD) technique. To achieve quantitative information, applying the sum rules to the dichroic signal of magnetic anisotropic materials is accurately discussed. Finally, the orbital-to-spin moment ratio of α-MnAs along the easy, hard, and intermediate magnetic axes is estimated by EMCD and compared to implemented density functional theory (DFT) calculations. The influence of the magnetocrystalline anisotropy is locally demonstrated. This work in particular illustrates the feasibility of the EMCD technique for in situ experiments, and proves its potential to explore the anisotropy of magnetic materials.