© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim The Bragg reflections of 1-decyl-3-methylimidazolium chloride ([C10mim][Cl]), a room-temperature ionic liquid, are observed in a lowly scattered wavevector (q) region using high-pressure (HP) small-angle X-ray scattering methods. The HP crystal of [C10mim][Cl] was characterized by an extremely long periodic structure. The peak position at the lowest q (1.4 nm−1) was different from that of the prepeak observed in the liquid state (2.3 nm−1). Simultaneously, Bragg reflections at high-q were detected using HP wide-angle X-ray scattering. The longest lattice constant was estimated to be 4.3 nm using structural analysis. The crystal structure of HP differed from that of the low-temperature (LT) crystal and the LT liquid crystal. With increasing pressure, Bragg reflections in the high-q component became much broader, and were accompanied by phase transition, although those in the low-q component were observed to be relatively sharp.