Cu electroless plating is one of the most promising methods to form a seed layer in TSV filling by Cu electroplating for 3-D integration. However, for increasing aspect ratios of TSVs, the uniformity of the Cu thickness at the sidewall of TSVs becomes poorer due to consumption of Cu ions and reducing agents near the entrance of TSVs. In this study, we have studied the addition of PEG-thiol to an electroless Cu plating bath with a reducing agent of glyoxylic acid. With this method we achieved near-conformal deposition of Cu for TSVs with diameters of 4 and 10 μm and 40 μm depth. The Cu thickness of the TSV sidewall remained constant with depth even for the high aspect ratio TSVs. This method is very effective for a formation of a thin Cu seed layer prior to the Cu electroplating process of TSV-filling.