We have investigated stability of a new kind of boron clusters in silicon crystal using the first-principles local density functional approach. In the calculation, we used three types of hydrogenterminated silicon clusters; one of these types included the four interstitial cluster I4 as a complex defect. According to our calculation, we found that a new kind of boron clustering of n-borons substituted I4 (n < 3) became most stable when the ratio r = N(Bi) / (N(Sii) + N(Bi) in a confined area was less than or equal to 1/2, where N(B) is a number of interstitial boron atoms and N(Sii) is that of interstitial silicon atoms. On the other hand, when r was greater than 1/2, the l4 would be destroyed to create the [100] split boron pair, the [110] split silicon pair, and/or the substitutional boron and interstitial silicon pair.