To serve as a practical solution which seems still unavailable to the problem of estimating the period Ti of natural vibration of reinforced concrete floor slabs, an approximate approach is herein proposed including the effect of their cracking and time-dependency. Admittedly the estimation is a major consideration in their serviceability limit state design. Consequently it has been clarified that Ti is proportional to the square root of long-time slab deflection with the proportional constant satisfactorily expressed in function form in terms of the ratio of the long-time value to the elastic value of slab deflection. Hence in practice Ti is readily predictable from a long-time deflection which may be estimated by the AIJ' s or other prediction methods.