In this paper, Nyquist diagrams of cyelotron instabilities caused by plasma streams are analysed. The calculation method on instability criterions of ns(the electron density of plasma stream), n0(the electron density of static plasma), B(the applied magnetie field strength), h(the wave number), V(the stream velocity) and T(the temperature) are developed. The numerical results of hydrogen plasma are given. Finally, the cyclotron and electrostatic instabilities caused by two plasma streams are also analysed.