A new method for fabricating ultrafine grooves with a high depth to width ratio is proposed. The main feature of this method is to use V‐shaped SiO2grooves as etching masks. These grooves are prepared by depositing a SiO2film onto the patterned substrate employing electron cyclotron resonance (ECR) plasma deposition, followed by removing the sidewall deposited ECR‐SiO2films with buffered HF solution. The gap width formed at the bottom of the V grooves were controlled by varying the HF etching time. Fine grooves having 0.2 μm or narrower widths were successfully fabricated into Si substrates and Ta films. Suppressing the localized undercutting is vital for fabrication of nanometer scale structures.