The brown seaweed Ecklonia cava is known to be a rich source of phlorotannins that have diverse biological activities. Among the phlorotannins in E. cava, concentrations of dieckol and phlorofucofuroeckol-A, which were identified as major active components, were determined in different parts of the tissue. We compared the efficacy of different pretreatments for their extraction. A high-performance liquid chromatography (HPLC) method to determine phlorotannin concentrations showed good accuracy (92.64 and 94.02%, respectively), precision (3.92 and 3.94%, respectively), and linearity (r>0.996). Mature thalli contained 1.5-fold more dieckol (1.82 mg/g-dry tissue) than young thalli. In the tissues of E. cava, blade tissue contained more phlorotannins than the stipe or holdfast. Among differently dried thalli, approximately 90% or more dieckol and phlorofucofuroeckol-A were extracted from shadow-dried tissue as compared with lyophilized tissue. In sun-dried and oven-dried thalli, approximately 60% of the phlorotannins were extracted. Thalli washed with fresh water, boiled thalli, and steamed thalli showed reduced extraction of the compounds.