The generation and evolution of the surface oxide layer of boron were thoroughly investigated due to the key role of the oxide layer in ignition and combustion of amorphous boron (B). Samples in different oxidation degrees were obtained by heating B particles until 600, 650, and 700 degrees C, using a temperature programmed thermobalance. A dual beam focused ion beam micro/nanofabricator was used to etch and cut the samples into thin slices (ca. 327 nm). The slices were observed under a scanning transmission electron microscope, accompanied with energy dispersive X-ray analysis. During the thermal oxidation process, B particles initially lost mass through dehydration. Then they began to get oxidized and gain weight markedly. The sample surface became more rough as the final temperature increased. Two different reaction modes took place in sequence during the thermal oxidation of the samples. Below 650 degrees C, the oxidation reaction occurred only on the surface of the particle (the surface reaction mode). However, when the samples were heated to 700 degrees C, the particle interior was also involved in the reaction (the global reaction mode), and a large number of pores were formed. The O content of the initial surface oxide layer was fairly high. The thickness distribution was uniform (average thickness 148.1 nm) and the two edges were both smooth. During the heating, the oxygen content of the surface oxide layer increased after an initial decrease. The average oxide layer thickness increased and the thickness distribution became irregular and unequal. The sample heated until 700 degrees C had an average surface oxide layer thickness of 379.3 nm, and the thickness span reached 354.3 nm. During the global reaction process (700 degrees C), the oxidation degree within the interior of the particle was lower than that on its surface. In the particle interior, pores near the center were smaller than those close to the edge, whereas the oxidation degree was uniformly distributed. Results in this work provide a deeper understanding of the surface oxide layer, which can potentially help improve the ignition and combustion features of B.