The　diode　effect　means　that　carriers　can　only　flow　in　one　direction　but　not　the　other.　While　diode　effects　for　electron　charge,　spin,　or　photon　have　been　widely　discussed,　it　remains　a　question　whether　a　chiral　phonon　diode　can　be　realized,　which　utilizes　the　chiral　degree　of　freedom　of　lattice　vibrations.　In　this　work,　we　reveal　an　intrinsic　connection　between　the　chiralities　of　a　crystal　structure　and　its　phonon　excitations,　which　naturally　leads　to　the　chiral　phonon　diode　effect　in　chiral　crystals.　At　a　certain　frequency,　phonons　with　a　definite　chirality　can　propagate　only　in　one　direction　but　not　the　opposite.　We　demonstrate　the　idea　in　concrete　materials　including　bulk　Te　and　alpha-quartz　(SiO2).　Our　work　discovers　the　fundamental　physics　of　chirality　coupling　between　different　levels　of　a　system,　and　the　predicted　effect　will　provide　a　new　route　to　control　phonon　transport　and　design　information　devices.