The final goal ofthis study is to develop a buoyancy control device for underwater robot utilizing phase change of materiai ． A conventiQnal underwator robot and a conventional submarine a 〔 加 st buoyancy by discarding a set of iron balls or seawater in other place， which is harmfUI to the environment 、 Therefbre ， we proposed new buoyancy control method utilizing phase change of material 」 n thc prcvious papers ， we used paraffin wax ． Howevor， its melting point is high ( approximately 47℃ ) ． Therefbre ， we must loQk 負 )r a new material that has lower melting point， and not harrnfu1 to thc cnvironment ． We manufactured a new buoyancy mechanism to test materials ． Finally ， we focused on lard and 行nd out that lard has lower melting point( approximatcly 25 ℃ ) with high density difference betwccn liquid and solid phases and thus response faster than para 幵in wax ． Key iferds：Underwatcr robot ， Buoyancy control ， Rubber stretch ， Lard