Total mass and total grain size distribution of TB tephra, a sub-plinian deposit of the 1986 eruption of Izu-Oshima, Japan, were estimated. Based on field survey and granulometric analysis, isopleth maps of mass loading per unit area were made for each size class of one phi interval. It was found that there exists a relationship between an isopleth (C) and area of the isopleth (A) which can be expressed as; ln C=-k Sqrt A. From this relation, total mass of each size class was obtained and total mass of the deposit was calculated from their sum. Obtained total mass of TB tephra is 3.6×10⁹ kg. TB tephra is very rich in coarse population compared with major plinian products. Scoria larger than 2 mm accounts for about 90% of the total deposit. Estimations of masses of fine fractions seem to be performed precisely owing to their large k and the fact that even for the finest fraction, highest concentration of fallout remains around the vent. Distal deposit which falls on the sea is restricted its maximum to assume k on the sea is higher than that of plinian eruption. This assumption seems to be reasonable because plinian eruption has a much smaller k presumably due to a much higher expansion rate of umbrella cloud. The obtained maximum estimation of the total mass of TB tephra is up to 1.4×10¹⁰kg.