  By analysing the data acquired from the Parkes 64-m radio telescope at 1369
MHz, we report on the phase-stationary non-drift amplitude modulation observed
in PSR J1048-5832. The high-sensitivity observations revealed that the central
and trailing components of the pulse profile of this pulsar switch between a
strong mode and a weak mode periodically. However, the leading component
remains unchanged. Polarization properties of the strong and weak modes are
investigated. Considering the similarity to mode changing, we argue that the
periodic amplitude modulation in PSR J1048$-$5832 is periodic mode changing.
The fluctuation spectral analysis showed that the modulation period is very
short (~2.1 s or 17 P1), where P1 is the rotation period of the pulsar. We find
that this periodic amplitude modulation is hard to explain by existing models
that account for the periodic phenomena in pulsars like subpulse drifting.
