Nonreciprocal wave propagation typically requires strong nonlinear materials to break time reversal symmetry. Here, we utilized a sonic-crystal-based acoustic diode that had broken spatial inversion symmetry and experimentally realized sound unidirectional transmission in this acoustic diode. These novel phenomena are attributed to different mode transitions as well as their associated different energy conversion efficiencies among different diffraction orders at two sides of the diode. This nonreciprocal sound transmission could be systematically controlled by simply mechanically rotating the square rods of the sonic crystal. Different from nonreciprocity due to the nonlinear acoustic effect and broken time reversal symmetry, this new model leads to a one-way effect with higher efficiency, broader bandwidth, and much less power consumption, showing promising applications in various sound devices. DOI: 10.1103/PhysRevLett.106.084301 PACS numbers: 43.35.+d, 42.79.Dj, 43.25.+y Electrical diodes, due to their capability of rectification of current flux, have significantly revolutionized fundamental science and advanced technology in various aspects of our routine life. Motivated by this one-way effect of electric currents, considerable effort has been dedicated to the study of the unidirectional nonreciprocal transmission of electromagnetic waves, showing important promise in optical and rf communications [1][2][3][4][5][6]. The realization of such nonreciprocal and unidirectional propagation requires either a broken time reversal symmetry [1-4] or a broken spatial inversion symmetry [5,6] in the artificial photonic structures (e.g., photonic crystals).Sonic crystals (SCs), in an analogy with the electronic and photonic band structures of semiconductors and photonic crystals, have shown promising impacts in acoustic devices and applications that can efficiently trap, guide, and manipulate sound [7][8][9][10][11][12][13][14][15]. In the past two decades, with rapid developments in SCs ranging from engineering of band structure for bulk acoustic waves to design of acoustic grating for surface waves, a series of fascinating acoustic effects are consequently demonstrated, such as acoustic band gaps [8,9], negative refractions [10][11][12][13][14], and extraordinary transmission [15]. It is therefore expected, with a sophisticated SC design, that the exotic properties of SCs can lead to more counterintuitive sound manipulation, for example, the realization of acoustic diodes that can break down the conventional transmission reciprocity [16][17][18][19]. Similar to electromagnetic wave, sound usually propagates reciprocally back and forth along a given path. Unidirectional flux transmission requires considering the breaking of parity and time symmetry simultaneously in uniform media [20] that do not typically exist in nature. Therefore, SCs are currently considered good candidates to implement nonreciprocal and unidirectional sound propagation. The previous studies proposed the utilization of acoustic nonlinear effec...