  Quasi-periodic oscillations in solar flaring emission have been observed over
the past few decades. To date, the underpinning processes resulting in the
quasi-periodic oscillations remain unknown. In this paper, we report a unique
event that exhibits both the long-duration quasi-periodic intensity
oscillations of flare loops and the quasi-periodic slipping motion of ribbon
substructures during a C9.1-class flare (SOL2015-03-15-T01:15), using the
observations from Solar Dynamics Observatory and Interface Region Imaging
Spectrograph. The high-temperature flare loops rooted in the straight part of
ribbons display a "bright-dim" intensity oscillation, with a period of about
4.5 minutes. The oscillation starts just after the flare onset and lasts over 3
hours. Meanwhile, the substructures within the ribbon tip display the
quasi-periodic slipping motion along the ribbon at 1400 \AA images which has a
similar periodicity to the stationary intensity oscillation of the flare loops
in the straight part of the flare ribbons. We suggest that the quasi-periodic
pattern is probably related to the loop-top dynamics caused by the reconnection
outflow impinging on the flare loops.
