An improved method of preparing optical fiber preforms and optical fibers by so called rod-in-tube method, wherein before collapsing a tube to heat-adhere to a rod, a specific glass surface treating agent and oxygen gas are flowed through the clearance between the rod and the tube maintained at a high temperature, whereby an optical fiber preform free from imperfections at the interface between the rod and the tube can be produced so as to give optical fibers of low-loss. As the above glass surface treating agent, those materials are employed which satisfy the following criteria: (i) the hydrogen content thereof being not more than about 1% by weight, and (ii) the substances, produced therefrom in the presence of oxygen gas at a high temperature, having a boiling point or sublimation point of not more than the temperature required for collapsing the tube.