Abstract: Three near-isogenic lines (NILs) under the genetic background of Q30（CsSUN/CsLNG1），including QK1.2-S（Cssun / CsLNG1），QK2.1-S（CsSUN / Cslng1），QK1.2+2.1-S（Cssun/Cslng1），were used in this study. Morphological observation showed that fruits of Q30 are much slender than those of QK1.2-S and QK2.1-S. Besides，Q30 also had smaller stem diameter and higher plant height. Histological observation of fruits at different developmental stages indicated that the fruits of Q30 had the smallest cell size and the highest cell density when compared with QK1.2-S，QK2.1-S and QK1.2+2.1-S. Endogenous hormone test of fruits showed that the BR/ABA and GA3/ABA levels of Q30 were significantly lower than QK1.2-S，QK2.1-S and QK1.2+2.1-S at the stage of six-day before anthesis and this difference gradually decreased with the process of fruit development. While, no significant differences were found for ZT/ABA and IAA/ABA levels among these materials. At the stages of anthesis and six-day before anthesis，the relative expression of CsSUN in Q30 was significantly higher than that of QK1.2-S and QK1.2+2.1-S. The expression level of CsLNG1 in Q30 was also higher than that of QK1.2-S and QK1.2+2.1-S. However, the difference gradually reduced with the development and growth of fruit. At the early development stage，the expression levels of cell expansion related genes Csa1G422480（xyloglucan endotransglucosylase gene）and Csa6G014540（expanded protein gene），BR biosynthesis related gene Csa1G524640 and GA3 regulation involved gene Csa3G872170 were significantly lower in Q30 when compared with QK1.2-S，QK2.1-S and QK1.2+2.1-S. However，the expression levels of Csa1G422480，Csa6G014540，Csa1G524640 and Csa3G872170 in Q30 were gradually increased as the growth of fruit，and even significantly higher than QK1.2-S，QK2.1-S and QK1.2+2.1-S at the later stages. In summary，the slender fruits of Q30 are due to the inhibitory effect of CsSUN and CsLNG1 on fruit cell enlargement in the early stage，which resulted smaller cell size and higher cell density. When fruit development reaches to the rapid growth period，the inhibitory effect of these two genes on cell size is weakened，and the fruit cells gradually expand，which promotes the rapid growth of fruit size.

Key words: cucumber, fruit size, CsSUN, CsLNG1