Flow boiling instabilities in a single microchannel with a low aspect ratio were studied by taking account of the effects of inlet temperature，mass flux，heat flux and inclination with methanol as the working fluid.Three types of instabilities were found over wide parameter ranges：flow excursion static instability，pressure-drop type oscillation，and superimposed instability of pressure-drop type and density wave type oscillations.Due to the large ratio of the channel length to the hydraulic diameter and the upstream compressible volume，the pressure-drop type oscillation occurred in the negative gradient region of the demand curve of pressure-drop versus mass flux.The influences of inlet temperature，inclination，mass flux and heat flux on flow instability were analyzed.It was demonstrated that whether the pressure-drop type oscillation occurred or not depended on inlet temperature，mass flux and heat flux.The range in which the pressure-drop type oscillation occurred was presented with outlet mass quality as control parameter.