Abstract: Despite the dominant role of electrons and photons as information carriers, magnons–the collective spin excitations–have emerged as a very promising candidate and attracted intensive attention recently. Among all the magnon media, yttrium iron garnet (YIG) distinguishes itself for its extremely low spin damping which leads to a very long coherence time for the magnon. In this talk, I will discuss the coherent interaction between magnon and microwave photon in a hybrid system containing a three dimensional microwave cavity and a YIG sphere. The large spin density in YIG, which significantly enhances the magnetic dipolar interaction between the magnon and the microwave photon, allows us to achieve strong and even ultrastrong magnon-photon coupling. Other coherent interactions, such as magnetically induced transparency and Purcell effect, have also been achieved, showing the great flexibility of our system. Such magnon-photon coupling provides us new possibilities of manipulating cavity photons, with which we successfully demonstrated a novel magnon gradient memory that can store information in the magnon dark modes with high efficiency. Our results is a crucial step toward utilizing magnon as a information transducer to interconnect different systems including microwave, optics and mechanics.