Today, the transportation and construction industry commonly use beams and prestressed concrete floors. The research of prestressed concrete beams and slabs often focuses on the static problem. Specific characteristics of the structure, such as natural frequencies, vibration modal functions and drag coefficients are parameters that have great influence on the responses of structural dynamics. They are used in the analysis, design and building inspection. Moreover, the research of vibration control for buildings under dynamic loads, which natural frequencies and vibration modal functions are two of the very significant parameters that need to be determined. Therefore, this research focuses on identifying natural frequencies and vibration modal functions of prestressed concrete beam with different boundary conditions. The research has established characteristic equations to determine natural vibration frequencies and modal functions. Based on that, using the Newton- Raphson numerical method by Matlab software to analyze the problems.