Native arenga starch has infirmity physicochemical properties so that its use is limited to functional food. Dual modification is a starch processing to improve physicochemical properties of starch. The objective is to determine the optimum concentration of sodium trimetaphosphate (STMP) based on the physicochemical properties of the phosphate butyrate arenga starch (PBAS) from dual modification of butyrylation and phosphorylation. The method of making PBAS is using butyric anhydride 5% (w/v) and STMP concentration variation consisting of 2%, 4%, 6%, 8%, 10% and 12% (w/w) of the starch weight repeated three times and native arenga starch as a comparison. Physicochemical properties analyzed were percent butyryl, degree of substitution (DS), phosphate content, binding of butyrate phosphate with fourier transform infrared (FTIR), water, starch, amylopectin, amylose and ash contents. The results showed that the optimal STMP concentration range of 4-6% based on percent butyryl, DS and phosphate content that produce PBAS suitable for functional food ingredients. Binding of the functional group of PBAS formed a new peak in the FTIR spectra at wave number 1365.60 cm-1 which indicates the presence of phosphate (P=O) groups of STMP on starch molecules. The water content of PBAS decreased with increasing STMP concentrations. The amylose, amylopectin and ash contents of PBAS were changed after the process of butyrylation and phosphorylation. PBAS is potential as a functional food ingredient due to its butyrate and phosphate contents.