We propose that the emissions of an advection-dominated accretion flows (ADAF) disc are the seed photons upscattered to x-ray and TeV γ-rays in Mkn 501 and the instability of an ADAF disc may explain the 23-day quasi-periodic oscillation period observed in the x-rays and TeV lightcurves of Mkn 501. In this model, the ADAF emissions of optical and x-rays go into the jet and are up-scattered to high energies by relativistic electrons. In this process, the instability occurring in ADAF results in the quasi-periodic variation in the seed photons and therefore causes the variation in high energy x- and γ-rays.