An organic silazane polymer is prepared by passing a silazane compound in por form through a hollow tube heated at 400.degree. to 700.degree. C. for activation and thermally polymerizing the silazane compound in a liquid phase. Inorganic fibers can be prepared from the organic silazane polymer by melt spinning, infusibilizing, and sintering. An apparatus for preparing an organic silazane polymer is also provided, comprising a heated reactor vessel for evaporating and thermally polymerizing a silazane compound, a heated hollow tube in flow communication with said reactor vessel for receiving the silazane compound vapor and heating it at 400.degree. to 700.degree. C. for activation, and a conduit, with a condenser for condensing the vapor, for feeding the condensed silazane compound back to the reactor.