This paper studies how sex imbalance in marriage market affects timing of the first marriage. As a major relaxation of the one-child policy in China during 1980s, the one-and-half children policy was implemented in some provinces which allows extra births conditional on the gender of the first child. Rural residents are allowed to have a second child if the first child is a girl, but are not if the first is a boy. Due to son preference at higher order births, this policy change leads to a higher sex ratio of males to females thus an excess supply of males in marriage market after 2000s. Based on data from CFPS 2014 and Chinese Census 2000, I estimate the effect of sex ratio on timing of the first marriage by exploiting exogenous variation of the policy implementation across regions and cohorts. From a difference-in-difference (DID) approach with mixture proportional hazard (MPH) duration models, I find that the first marriage is significantly delayed in regions with a high sex ratio for both males and females, but the effect is smaller and less significant for females.