Effects of body weight (1.022±0.821, 0.557±0.266 and 0.303±0.277)and salinity (S: 13, 18, 23, 28, and 33) on oxygen consumption rate and ammonia excretion rate of Mactra chinensis Philippi were studied by means of experimental ecology methods at water temperature 18℃. The results showed that (1) Individual size and salinity had significant effected on oxygen consumption rate (P<0.01),and interaction of the both factors had significant effected on oxygen consumption rate (O) (P<0.05).The relationship between O and dry weight of soft tissue (W) can be represented by the allometric equation R₀=aW^-b, where a ranges from 0.695 to 1.762, with mean of 1.449, and b ranges from 0.446 to 0.587, with mean of 0.542. (2) Individual size and salinity had significant effected on ammonia excretion rate (N) (P<0.01);ammonia excretion rate decreases as the increase of body size and there is a negative exponential regressive function R_N=a₀W^-b₀. (3) The relationships of O and N with W and S can be described by the following linear regression equations: R_O =2.111-1.817W+0.49S(R²=0.546,F=34.294,P<0.001);R_N=168.186-120.589W+1.734S (R²=0.561, F=36.418, P<0.001).