Polymethylsilsesquioxane (PMSQ) aerogels were synthesized by using methyltrimethoxysilane (MTMS) as precursor and dimethyl sulfoxide (DMSO) as solvent via sol-gel method and supercritical drying. The laws of gelation times of PMSQ aerogels were discussed by changing the amount of ammonium hydroxide (AH) and DMSO. PMSQ aerogels possess typical nano-porous structures. The gelation time is shortened by the increase of amount of AH, but is extended by the increase of amount of DMSO. As the DMSO content increases, the viscosity changeSis retarded, whereas the increase of AH results in the acceleration of the viscosity change. The short or circle Si–O–Si linkages are generated because of the influence of theSunhydrolyzed –CH₃ groups, which causes the aggregates of particles and phase separation. The uniform gels can be obtained by controlling the relationships between gelation time and the time of phase separation.