This study presents the calculation method of ultimate flexural moment of reinforced concrete (RC) beams using the bi-linear diagram and tri-linear diagram of non-linear model of concrete deformation according to the Vietnamese standard TCVN 5574:2018. In parallel, the four-point bending test was carried out on six RC beams with the dimension bxhxL=120x200x2,200 mm, made of concrete having the compressive strength of B25 class, divided into three groups with different ratios of longitudinal reinforcement at the bottom layer, 2Ø8, 2Ø10, and 2Ø12 steel bars, respectively. The theoretical results were compared with the experimental results obtained in order to select the appropriate calculation method, analyse the flexural behaviour of RC beams under static load as well as the effect of longitudinal reinforcement on their behaviour.