In this study, nonlinear vibrations of a slightly curved beam having arbitrary rising function are handled. The beam is restricted in longitudinal direction using elastic supports on both ends. Sag-to-span ratio of the beam, which is assumed to have sinusoidal curvature function at the beginning, is taken as 1/10. Beam being of Euler-Bernoulli type rests on Winkler elastic foundation and carries an arbitrarily placed concentrated mass. Equations of motion are obtained by using Hamilton Principle. Cubic and quadratic nonlinear terms have been aroused at the mathematical model because of the foundation and the beam's elongation. The Method of Multiple Scales (MMS), a perturbation technique, is used to solve the equations of motion analytically. The primary resonance case is taken into account during steady-state vibrations. The natural frequencies are obtained exactly for different control parameters such as supports' types, locations of the masses and linear coefficient of foundation. Frequency-amplitude and frequencyresponse graphs are drawn by using amplitude-phase modulation equations.