Abstract: In existing material tests of steel bars and experimental studies on reinforced concrete components, it is common to define the beginning of bar bending under compression as the onset of buckling through visual observation, which is arbitrary and inaccurate. Based on the experimental results of cyclic tension-compression tests of steel bars, the correlation of lateral deflections in two orthogonal directions and the variation of the total lateral deflection are studied. Combined with Rodriguez's suggestion which uses the strain difference of opposite faces (concave side and convex side) of a specimen, three determination methods are proposed to predict the onset of buckling, namely “observation method”, “critical lateral deflection” method and “critical variation of lateral deflection” method. They are based on visual observation, the measurement of total lateral deflection and its variation, respectively. The analysis of 35 cyclic loading tests of steel bars using the three methods indicates that a combination of the “critical lateral deflection” method and the “critical variation of lateral deflection” method can predict the onset of buckling more reasonably and accurately.