  We present the preliminary result of the calculation of $B_B$ parameter using
lattice NRQCD. The calculation is performed on a $16^3 \times 48$ quenched
lattice at $\beta=5.9$ with clover light quark. The use of lattice NRQCD
enables us to investigate the $1/m_Q$ correction from the static limit. The
observed mass dependence is well described by the vacuum saturation.
