A viable and low-cost strategy for fabricating high-aspect-ratio microstructures on silicon (Si) based on a judicious combination of flow-enabled self-assembly (FESA) and metal-assisted chemical etching (MaCE) is reported. First, polymer patterns were directly formed on a bare Si substrate in one step by FESA of polymers in a two-parallel-plate setup consisting of a fixed upper plate and a movable lower Si substrate that was placed on a programmable translation stage. The implementation of FESA to yield polymer patterns eliminates the complicated manipulation of polymer resist in conventional lithography methods. Subsequently, these polymer patterns were utilized as a stable etching mask in the MaCE step and exhibited a remarkable selectivity of 467:1 over Si during etching. Notably, such a combined FESA and MaCE strategy (i.e., a FESA-MaCE route) avoids the use of a hard mask which is necessary for the conventional plasma etching method. During MaCE, a layer of Au thin film was used as a catalyst and hydrogen peroxide-hydrofluoric acid solution was employed as the etching solution. Consequently, trenches and gratings on Si at a micrometer scale with uniform controllable geometry were successfully produced. The aspect ratio of microstructures was up to 16:1 and the lateral edge roughness was below 0.5 μm. The influence of processing conditions on the geometry of the etched structures was scrutinized through a comparative study. The geometry of the etched structures was found to effectively adjust their surface wettability in a continuous manner. Clearly, due to the ease of implementation and the batch processing capability, the FESA-MaCE strategy is promising in manufacturing a broad range of high-quality Si-based devices at low cost.