Stearoyl-ACP desaturase (SAD), which catalyzes the first desaturation reaction of the most common pathway of unsaturated fatty acid synthesis in plants, plays a key role in determining the ratio of saturated to unsaturated fatty acids. Using rapid amplification of cDNA ends (RACE) and a peanut cDNA library we had previously constructed, we isolated a 1499 bp cDNA of the SAD gene containing a 1218 bp complete open reading fragment (ORF). Sequence analysis revealed that peanut SAD had a high level of nucleotide and amino acid sequence homology to SAD from other plants. Characteristics of the deduced protein were predicted and analyzed using bioinformatic methods. Phylogenetic analysis showed that three different types of delta-9 fatty acid desaturases form separate clades, indicating that the three isozymes may have evolved independently. Expression analysis via real-time PCR indicated that expression levels of the SAD gene were markedly distinct in different peanut tissues and varieties. The results of the expression analysis in this study, combined with existing research, suggest that SAD may be involved in the regulation of plant seed growth and development.