An electron beam lithography method for printing and plating sub-50 nm isolated trenches with a high aspect ratio has been developed for the nanofabrication of magnetic thin film heads. To eliminate the issues of resist footing and resist residue in the narrow trench process, we put a thin dissolution layer of polymethylglutarimide (PMGI) as an undercoat layer between a seed layer and a resist layer. The undercoat dissolution layer competely cleared off the seed layer by the developer solution such that the sides of the narrow trench are vertical, particularly at the bottom of the narrow trench, thus facilitating plating the narrow trench with a high magnetic moment material. In this work, the narrow trenches were electroplated with both 1.0T NiFe and 1.8T CoNiFe. Three key issues in our trench process will be discussed here, including: 1) critieria for the selection of the undercoat dissolution layer materials; 2) processing conditions control , e.g. the thickness and the bake temperature of the dissolution layer to achieve vertical and smooth sidewalls; and 3) PEB delay on the narrow trench CD control, pattern degeneration, and the results from the resist top coat (RTC) experiments. With our new narrow trench process, we demonstrated the capability of fabricating narrow electrodeposited magnetic write structures with a CD of 35 nm in 0.35 μm resist (AR=10:1) and a CD of 30 nm in 0.25 μm resist (AR=8:1).