A quantitative method is used for determination of emodin in Rumex cyprius Marb., using high performance liquid chromatography. The plant parts: roots, stems, and leaves, were soaked in 95% methanol solution for 48 hours. Plant parts and standard emodin were eluted from C18 column with a mobile phase consisting of methanol: water (80%:20%volume/volume) at a flow rate of (2.0 milliliter / minute). The effluent was monitored with ultraviolet detector set at 258 nm. Standard curve for emodin was linear in the range of concentration 1.35- 45.0 ppm .The method was applied for determination of emodin in roots, stems and leaves of R. cyprius Marb. The results showed that the concentration of emodin in roots, stems, and leaves were 0.16%, 0.04%, and 0.30% respectively in dried plant sample. On the other hand the results obtained showed that emodin exhibits two absorption maxima at 410nm and at 510nm. The absorption peak at 410nm was found to decrease gradually by increasing pH, while the absorption peak at 510nm was found to increase gradually by increasing the pH.