A simple and ultrasensitive DNA biosensor was developed utilizing the color and size changes originating from Ligation Chain Reaction (LCR)-based gold nanoparticle assembly as an indicator. Using synthetic DNA of 10-fold serial dilutions as a target, the limit of detection (LOD) was 1.5 aM with a range of DNA concentrations from 0.01 to 1000 fM by UV-vis absorption and 0.1 aM in the range of 0.01-10,000 fM by dynamic light scattering (DLS). This developed method is low-cost, rapid, and much lower LOD for the detection of specific short DNA sequences, and the results can be directly determined either by the color or by the DLS. The capability of high-throughput detection with the aid of PCR amplification apparatus can also be realized.