Abstract: To yield grain size results of marine sediments reflecting variation of paleoenvironment and paleoclimate without distortion, biogenic carbonate and opal in samples should be effectively removed while terrigenous component should be undamaged during pretreatment procedure before measurement. To find the appropriate pretreatment method, marine sediments from South China Sea were treated with different dosages of hydrochloric acid (10%) or acetic acid (25%) and different dosages of NaOH and Na2CO3, then organic element analyzer, silicamolybdenum blue colorimetry and ESEM were used to evaluate the effects combined with the results of grain size measurement. The synthetic outcomes reveal that both of hydrochloric acid and acetic acid can eliminate biogenic carbonate well, however, because even a few dosage of the former can destroy terrigenous mineral component, the latter should be adopted during pretreatment procedure owing to its lower acidity; a large dosage of Na2CO3 still can't eliminate biogenic opal effectively but only a few dosage of NaOH can work well, so the latter should be used with restricted dosage to avoid destroying mineral component. According to biogenic content in marine sediments from SCS, the quantities of acetic acid (25%) and NaOH were determined and pretreatment procedure was established.