By first-principles calculations, we find that Ba[subscript 2]X(X=Si,Ge) hosts a topological semimetal phase with one nodal ring in the k[subscript x]=0 plane, which is protected by the glide mirror symmetry when spin-orbit coupling (SOC) is ignored. The corresponding drumheadlike surface flat band appears on the (100) surface in surface Green's function calculation. Furthermore, a topological-semimetal-to-insulator transition is found. The nodal line semimetal would evolve into a topological insulator as SOC is turned on. The topologically protected metallic surface states emerge around the [bar over]Γ point, which could be tuned into the topologically trivial insulator state by more than 3% hydrostatic strain. These results reveal a category of materials showing quantum phase transition between topological semimetal and insulator, and tunability through elastic strain engineering. ; National Basic Research Program of China (973 Program) (Contract 2012CB619402) ; Xi’an Jiaotong University. 985 Project ; National Science Foundation (U.S.) (Grant DMR-1410636) ; Solid-State Solar-Thermal Energy Conversion Center ; United States. Dept. of Energy. Office of Basic Energy Sciences (Award DE-SC0001299/DE-FG02- 09ER46577)