Abstract: During 1980-1984, It is found that the spotless flares are associated with eleven groups of microwave burst in solar radio data observed with the radiotelescopes at 9375 MHz, 3653 MHz and 2902 MHz. In this paper, the temporal behavior and peak-flux-density of the microwave bursts are studied. They are long duration, small flux and gradual rise-and-fall events. Source size of microwave bursts is similar to spotless flare source, they may occur in low coronal layer or in chromospheric high layer. Finally, their emission mechanism are confirmed as thermal bremsstrahlung from electrons with Maxwellian distribution.