Surface plasmon resonance (SPR) phase-shift interferometry (PSI) is a novel technique which combines SPR and modified Mach–Zehnder PSI to measure the spatial phase variation caused by biomolecular interactions upon a sensing chip. The SPR-PSI imaging system offers high resolution and high-throughout screening capabilities for microarray DNA hybridization without the need for additional labeling, and provides valuable quantitative information. The SPR-PSI imaging system has an enhanced detection limit of refraction index change, a long-term phase stability of π/100 in 30 min, and a spatial phase resolution of π/300 with 100×100 μm2 detection area. This study successfully demonstrates the label-free observation of 15-mer DNA microarray. © 2005 Society of Photo-Optical Instrumentation Engineers.