The earth has a 24-hour period, and that gives most living organisms a circadian clock which coordinates their internal biological events with external environmental signals. Recently, Arabidopsis circadian clock is shown to regulate plant stresses responses, such as drought, cold, pathogen and wounding. Salinity is one of worldwide problems on agriculture, and excessive Na+ greatly retards the plant growth and reduces yields of crops. However, the link between circadian clock and salt stress responses has not been investigated. In this study, Arabidopsis wild type appears to be highly sensitive to salt in day time compared to night time. This phenomenon can be explained by higher salt accumulation in day time than in night. Salt and drought responsive transcription factor RD29A is induced by salt higher during day than at night. Furthermore, stability of Na+ / H+ antipoter SOS1 transcript and subsequent SOS1 protein accumulation in SOS1 constitutive over-expression transgenic plants are much increased in day time than at night, when they were treated with salt. Thus, I propose the plant circadian clock is involved in controling appropriate time when plant responses to salinity stress by regulating expression of functional RD29A and SOS1.