Transcription by the nuclear factor of activated T cells (NFAT) is regulated by the frequency of Ca 2+ oscillation. However, why and how Ca 2+ oscillation regulates NFAT activity remain elusive. NFAT is dephosphorylated by Ca 2+‐dependent phosphatase calcineurin and translocates from the cytoplasm to the nucleus to initiate transcription. We analyzed the kinetics of dephosphorylation and translocation of NFAT. We show that Ca 2+‐dependent dephosphoryl ation proceeds rapidly, while the rephosphorylation and nuclear transport of NFAT proceed slowly. Therefore, after brief Ca 2+ stimulation, dephosphoryl ated NFAT has a lifetime of several minutes in the cytoplasm. Thus, Ca 2+ oscillation induces a build‐up of dephosphorylated NFAT in the cytoplasm, allowing effective nuclear translocation, provided that the oscillation interval is shorter than the lifetime of dephos phorylated NFAT. We also show that Ca 2+ oscillation is more cost‐effective in inducing the translocation of NFAT than continuous Ca 2+ signaling. Thus, the lifetime of dephosphorylated NFAT functions as a working memory of Ca 2+ signals and enables the control of NFAT nuclear translocation by the frequency of Ca 2+ oscillation at a reduced cost of Ca 2+ signaling.