Abstract: Sea buckthorn (Hippophae rhamnoides) is an economically important tree that grows well under dry and unfertile conditions and has berries with valuable bioactive compounds. Plant endogenous hormones (i.e., indole-3-acetic acid (IAA), gibberellin (GA₄), abscisic acid (ABA), and jasmonic acid (JA)) play an important role in regulating the growth and development of fruits. However, the dynamic contents changes of these hormones during fruit development in sea buckthorn have not yet been determined. In this study, we investigated two closely related sea buckthorn accessions with widely different oil contents. Fruits were harvested at different developmental stages of the two accessions on July 1, July 17, August 2, and August 18, and contents of four endogenous hormones (IAA, GA₄, ABA, and JA) were analyzed by high performance liquid chromatography-mass spectrum (LC-MS). The moisture contents in berries remained about 80%–85% throughout fruit development, which were beneficial to the relative stability of endogenous hormone concentration. The growth rates of oil contents were basically the same during different development stages. By contrast, oil contents began to differ between the two accessions during the middle and late periods of fruit development. Concentrations of IAA and JA were relatively high in the beginning, then decreased toward the end of fruit ripening. The concentration of ABA was relatively high in the middle and late stages of fruit development, whereas concentration of GA₄ was much lower; a high ratio of ABA to GA₄ was advantageous to oil biosynthesis and accumulation in the fruits. The results provide a theoretical basis for understanding the regulation of different endogenous hormones in sea buckthorn fruit development.