Recent observations reveal a similar frequency dependence f-b in the X-rays from stellar black hole candidates and active galactic nuclei (AGNs). Several X-ray sources have a steeper decline of the power spectrum, b=1.5-1.7 at f>1.0 Hz. Data from EXOSAT have shown that AGNs have a universal slope of b~1.55. These spectral dependencies give us information about the activity of accretion disks around compact gravity sources. To explain these power spectra and the activity of the accretion disks uniformly, several research groups hypothesize that many blobs exist on the disks and that the power spectra are explained by their activity. These accretion disk blobs are discussed in this paper. We show that accretion disk blobs cannot survive unless each of them is sustained by a tension that presumably originates from an external magnetic field. The diffusion mechanism of the magnetic field is also important, since the magnetic tension that sustains the blobs becomes ineffective when the strength of the magnetic field is smaller than a critical value. If we can determine this critical value, we will reveal something about the evolution of accretion disks.