The present invention relates to a method of isolating semiconductor devices enabling to prevent an active area from being reduced due to the increase of an isolation area by means of forming trenches, and includes the steps of forming a mask on a semiconductor substrate wherein the mask discloses field areas, forming a first and second trench in the field areas of the semiconductor substrate wherein the first trench has a larger size and a lower aspect ratio than those of the second trench and wherein the second trench has a smaller size and a higher aspect ratio than those of the first trench, depositing filling oxide on the mask and in the first and second trench by a method including characteristic of sputtering wherein the first and second trench are filled up with the filling oxide and a void is formed on a lower part of the second trench, and forming field oxide film by means of etching back the filling oxide to remain inside the first and second trench.