We have investigated the nanofabrication of three-dimensional (3D) chemical vapor deposited (CVD) diamond molds in Electron Cyclotron Resonance (ECR) oxygen ion beam etching technologies using polysiloxane n as an electron beam (EB) mask and a room-temperature (RT)-imprint resist material. The polysiloxane exhibited a negative-exposure characteristic and its sensitivity was 5.5 × 10 −5 C/cm 2 . The maximum etching selectivity of polysiloxane film against diamond film was 4.7, which was obtained under the following ECR oxygen ion etching conditions: ion energy of 400 eV, ion incidence angle of 0• , microwave power of 100 W, gas pressure of 1.4 × 10Pa and stage temperature of 24 • C. The diamond molds of cone and tetragonal pyramid dots were fabricated with polysiloxane mask in EB lithography technology using the RT-nanoimprint lithography (NIL) process. The dots are 500, 600, 700, 800, 900 nm in diameter and width respectively. The pitch between the dots is 2 µm, and each dot has a height of about 1 µm. It was found that the optimum imprinting pressure and its depth obtained after the press for 5 min were 0.5 MPa and 0.5 µm respectively. The resulting diameter of each imprinted polysiloxane pattern was in good agreement with that of the 3D-diamond mold. We carried out the RT-NIL process for the fabrication of diamond nanopatterns, using the 3D-diamond molds that we developed.