Abstract: 【Objective】Studying the formation of cucumber hollow traits,analyzing the correlation between hollow grades and phenotypic traits,so as to provide technical support for improving the quality and commerciality of cucumber fruits and the breeding of high-quality cucumber varieties.【Method】The North China fresh cucumber 9930 and the North American processed cucumber WI7120,which were significantly different in fruit hollow traits,were used as materials to observe and determine the changes in the microstructure and endogenous hormone content during the fruit development process.【Result】WI7120 showed obvious hollow character 15 d after pollination,which was the key period for cavity appearance,and the hollow grade increased to the highest level 30 d after pollination.Fruit length of 9930 and fruit diameter of WI7120 increased in a double S curve,fruit diameter of 9930 and fruit diameter of WI7120 increased in a single S curve. Except for ABA,the hormone content in WI7120 fruit was more than 9930,and GA>IBA>ZT>ABA.The GA content in each part of 9930 fruit showed a downward trend within 15 d after pollination and then increased,while that in WI7120 fruit decreased.IBA content in the carpel of WI7120 showed a decreasing trend 15 d after pollination,but the other parts showed an increasing trend. ZT and ABA contents increased after pollination. Observation of paraffin section indicated that the cells in the carpel of WI7120 fruit grew oblong 15 dafter pollination.The cells in the carpel were enlarged much larger than those in the flesh. Correlation analysis showed that the hollow grade of WI7120 fruit was significantly positively correlated with peel GA and ZT contents(P<0.05),and hollow grade had extremely significantpositive correlation with fruit length and diameter(P<0.01).【Conclusion】The IBA and GA in the carpel of WI7120 fruit are higher than that in the flesh,which might be the root cause of the hollowness in WI7120 fruit.By promoting cell division,ZT acts on fruit cell density to control fruit transverse diameter and further affect fruit hollow grade.