The Wuschel related homeobox (WOX) family proteins are key regulators implicated in the determination of cell fate in plants by preventing cell differentiation. In this study, we identified a WOX protein from rice, named RiceHB4, which is involved in early flowering under different day length conditions. RiceHB4 is a member of the most conserved WOX clade in plants, WOX13, which affect root and flower development in Arabidopsis. RiceHB4 is highly expressed in germination seeds, roots, and mature leaves. During flowering stage, it is abundant in flag leaf then accumulated in flower after pollination. When over-expression of RiceHB4 under rab21, a drought and seed specific promoter, the early flowering phenotype was observed through T1 and T2 generations both in the field and green house conditions, on the main harvest of the year (May to Oct, long-day condition). Besides, there is an over-expression line under constitutive promoter that stably flowers earlier than wild-type in short-day (winter-early spring) but not long-day condition, which suggests RiceHB4 may involve in photoperiod or light signaling response. The early flowering phenotype of different transgenic plants, which were transformed with different promoters, in different conditions suggests that HMB4 may have more than one function that promotes the early flowering in rice.