Our Galaxy is filled with cosmic rays, but the origin of PeV cosmic rays have been unknown for a long time. Recently, LHAASO discovered mysterious sub-PeV gamma-ray sources without any obvious counterparts in other wavelengths, and origins of these gamma-ray sources are also unknown. In this talk, we propose that isolated stellar-mass black holes (IBHs) wandering in molecular clouds can be the source of PeV cosmic rays and un-ID LHAASO sources. The accretion flow onto black holes can be magnetically arrested disks (MADs), where magnetic reconnection can accelerate high-energy particles via magnetic reconnection. We show that MADs around IBHs can accelerate cosmic-ray protons up to PeV energies and their production rate can be consistent with the PeV cosmic rays observed on Earth. These PeV cosmic rays interact with gas in molecular clouds, and we should see sub-PeV gamma-rays from molecular clouds where IBHs are embedded. This scenario can explain the hard spectral feature in TeV bands seen in some LHAASO unidentified sources.