A nano-diamond is an attractive neutron reflection material below cold neutron energy. The total neutron cross section of a nano-diamond produced using the detonation explosive technique was derived from a neutron transmission measurement over the neutron energy range of 0.2 meV to 100 meV because total neutron cross section data were not available. The total cross section of a nano-diamond with particle size of approximately 5 nm increased as the neutron energy decreased to 0.2 meV. It was approximately two orders of magnitude larger than that of carbon at 0.2 meV. The contribution of inelastic scattering to the total cross section was shown to be negligibly small at neutron energies of 1.2, 1.5, 1.9, 2.6, and 5.9 meV in inelastic neutron scattering measurements. Moreover, small-angle neutron scattering measurements of the nano-diamond showed a large scattering cross section in the forward direction at low neutron energies.