We have designed the backward wave oscillator operating at 24 GHz. From the research which sees researches in the goal which will design and will produce K-band BWO where is a backward wave oscillator which departs from cycle prisoner 24 GHz until now is higher. To design Chrencov instibility and branch of family used a slow cyclotron instibility. Calculation used a dispersion relation and in order for as the box over-flow not to happen, a asymtotic expansion. Used a beam mode and a waveguide mode and axial symmetry and expense used in compliance with sattle point interpreted the relationship of axial symmetry.