Significant progress has been achieved in the use of heterosis in soybean and several soybean hybrids have been released in China. However, broad use of hybrid soybean seed is limited due to low seed setting of female parents.  Breeding cytoplasmic male sterile (CMS) lines with high out-crossing rate is necessary to solve the problem.  The objective of this study was to determine the relationship between out-crossing rate of CMS lines and their nectar secretion.  The daily nectar secretion rhythm, meteorological effect on nectar secretion, and differences in nectar secretion among genotypes and years were investigated in 27 soybean CMS lines (A-lines) with their maintainers (B-lines) and restorers (R-lines).  The correlation between out-crossing rate of CMS lines and nectar production was also evaluated.  Nectar secretion had diurnal variation.  Secretion initiated at about 06:00 for most materials and reached a peak at 07:00–08:30 after flower opened, then the nectar secretion decreased gradually.  A sub-peak appeared at about 13:00, while the nectar could not be detected at 17:00.  Nectar secretion was greatly influenced by the weather conditions.  The amount of nectar secretion increased gradually over time during periods of high temperature and no rainfall for several days.  Rainy weather and low temperatures inhibited nectar secretion.  There were obvious variations of nectar amount among different genotypes tested.  Significant nectar variation within a genotype among years was also observed, and the highest nectar secretion was 3-fold higher than the lowest.  The amount of nectar secretion from R-lines was significantly higher than that of A- and B-lines.  There was no significant difference in nectar secretion between A- and B-lines.  A- and B-lines with higher out-crossing rates secreted more nectar.  The amount of nectar secretion of A- and B-lines were significantly positively correlated with the out-crossing rate of A-lines.