In this paper, a series of laboratory experiments was conducted on a large–scale Prestressed Concrete I (PCI) beam with a straight unbonded tendon. In specific, the simply supported PCI beam was subjected to free vibration tests with different prestress loads. Then, the results of numerical modeling, based on the Euler–Bernoulli beam theory, were compared with the experimental data. It was proved that the fundamental frequency of prestressed concrete beams with straight unbonded tendons is unaffected by the increasing applied prestress force, when these members are subjected to small second–order effects. Moreover, the compression–softening behavior is prominent even if its classical decreasing trend is modified by the variation of the elastic modulus of concrete with time. The experimental results also confirmed that the fundamental frequency is not a reliable indicator to detect the prestress loss in concrete beams with straight unbonded tendons.