This paper describes a luminance based rate control (RC) model for high dynamic range (HDR) video. A novel mathematical relationship between luminance and bit allocation of a coding tree unit (CTU) is presented. By adjusting the existing RC algorithm through the proposed model, a better balance between dark and bright areas can be achieved and -4.4% gains can be obtained in terms of average BD-Rate (tPSNR-XZY). Moreover, subjective assessment also shows that, compared with the existing RC model, the proposed method can convey a wider range of perceptible shadow and highlight more details.