overgrowth of soybean plants and thereby brings about lodging, decrease in pod number, damage by disease and insect pest, etc. This experiment was carried out to investigate optimum decapitation method in vinyl mulching soybean cultivation. Sinbuseoktae, local soybean variety was sown on May 25 and June 15 in 2009. Decapitation treatments(Non-decapitation, V8 stage-stem, V8 stage-apex, Flowering initiation stage, V8 stage-Stem+Flowering initiation Stage) were practiced on major growth stage on May25 seeding. Lodging did not occur in apex decapitation at V8 stage while other treatments had severe lodging. NAR from R5 to R7 stage was also highest in apex decapitation at V8 stage, which meaned optimum plant organ structure in utilizing light. On the other hand, flowering date and maturity date among treatments were similar. Seed yield in apex decapitation at V8 stage was highest with 274kg/10a, followed by flowering initiation stage with 249kg/10a. The apex decapitation at V8 stage had 26% yield increase as compared with the control of 216kg/10a due to heavy 100-seed weight.