The sulphur in plant tissues was transformed into SO4(2-) by digestion and it reacted on BaCrO4 to produce CrO4(2-) by exchange reaction, then the sulphur quantity in the plant was indirectly determined by flame atomic absorption spectrometry (FAAS). Various conditions affecting the quantative determination of SO4(2-) with this method were studied and the interference of PO4(3-), which existed in the digestion solution, could be overcome by addition of sufficient calcium. The results obtained by this method are consistent with those obtained by ICP-AES.