The diffraction phenomena of X-rays in the case ΘB≅π/2 are studied on the basis of dynamical theory. The angular width of diffraction for ΘB= π/2, 2√|χh| , is about 103 times as broad as the one for ΘB ⪡π/2, 2|Xh|/sin ΘB . Similar characteristic phenomena are expected for electrons. The π/2 Bragg angle diffraction would be utilized as X-ray resonator.