The laser-diode self-mixing interferometry can be employed to measure the sizes of nanoparticles. The standard nanoparticles whose sizes are labeled as 180, 100 and 60 nm are measured and the corresponding power spectra of the self-mixing signals are obtained with a series of bandpass filters. As a comparison, the power spectral densities (PSD) are achieved from the Fourier transform of the self-mixing signals which are recorded in real time. With the inversion algorithm, the particle-size distributions are extracted from either the PSD or the power spectra with bandpass filters (PS-BPF). It is showed that the particle-size distributions obtained from these two methods agree with each other and consist satisfactorily with those measured by the commercial particle-size analyzer. The analysis on the illness condition of modal matrices of these methods indicates that the technique based on bandpass filters is better than that based on the PSD.