Active galactic nuclei are believed to be powered by accretion onto super-massive (M >~ 10^6 M_sun) black holes. In order to study the accretion physics, AGN evolution and relationship with host galaxy a reliable black hole mass estimation method is required. I will briefly review the Reverberation Mapping (RM) and Single Epoch Virial (SEV) black hole mass estimation methods for Type I AGN, and discuss the underlying assumptions and possible biases affecting the mass estimates. Then I will introduce a new black hole mass estimation method based on the assumption that accretion occur through a standard Shakura&Sunyaev (1973) accretion disk. I will apply the method to a sample of radio-loud narrow-line Seyfert 1 galaxies and show that the resulting black hole mass estimates are significantly greater than SEV ones. I will discuss the reliability of these estimates, and the implications on the interpretation of the class of narrow-line Seyfert 1 galaxies in the framework of the AGN unified model.