The importance of DNA sequence design for reliable DNA computing is well recognized. In this paper, we describe a DNA sequence optimization system NACST/Seq that is based on a multiobjective genetic algorithm. It uses the concept of Pareto optimization to reflect many realistic characteristics of DNA sequences in real bio-chemical experiments flexibly. This feature allows to recommend multiple candidate sets as well as to generate the DNA sequences, which fit better to a specific DNA computing algorithm. We also describe DNA sequence analyzer that can examine and visualize the properties of given DNA sequences.