The binding characteristics of meso-tetra-(3, 5-dibromo-4-hydroxphenyl) porphyrin [T(DBHP)P] with calf thymus DNA (ct DNA) in pH 4.92 HAc-NaAc buffer solution were studied by the techniques of spectrophotometry. At the maxium absorption wavelength for T(DBHP)P, the decrease in the absorbance was linear with the amount of ct DNA. The experiments indicated that under optimum conditions, ct DNA obeys Beer's law in the range of 0.20-1.80 microg x mL(-1), and the limit of detection is 0.024 microg x mL(-1). Tween-80 microemulsion was commended in the experiment for better sensitivity. Furthermore, the association number is also determined by molar ratio method. The interactional mechanism between ct DNA and (T(DBHP)P was investigated as well.