Ascidian, Halocynthia roretzi, mainly inhabiting the subtidal rocky bottom as a sessile form, has been favored a tasty sea food in abundance along the east and south coast of Korea. The study was attempted to establish the basic data for evaluating the processing suitability of ascidian. The height, width, total weight, integument weight and edible portion of the ascidian were $9.2{sim}12.2cm,;4.7{sim}6.1cm,;100.2{sim}163.0g,;18.2{sim}41.4g$ and $20.2{sim}62.0g$, respectively. The moisture content, which showed a minimum value as low as $80.0\%$ in October. On the contrary, glycogen content of edible portion revealed the maximum value, $3.1{sim}4.1\%$ from July to August. The protein and fat showed a similar change to glycogen. The level of ash was rather constant throughout the period, being $2.2{sim}3.8\%$. Among the 7 kinds of mineral analyzed in all the samples of edible portion and integument, Na, K, Mg and Ca contents were predominent in order being more than $96.14{sim}99.10\%$, of the total ash. In the amino acid composition of ascidian, the predominent ones were asparagine, glutamic acid, taurine, aspartic acid, proline, and lysine in order and the sum of these amino acids occupied $85.0\%$ of the total free amino acid while methionine and arginine were poor.