In higher plants nitrate reductase genes(nia) has a crucial function for nitrate assimilations. I had reported nia 1 gene from rice( Oryza sativa). Rice nia1 gene has an open reading frame of 916 amino acids. The structural gene is composed of four exons and three introns. Many nucleotide sequences of nia genes have been published so far from Arabidopsis thaliana, Nicotiana sylbestris, Nicotiana tomentosiformis, Cucurbita maxima , Spinacia oleracea, Lycopersicon esculentum, Phaseolus vulgaris, Hordeum vulgare, and Zea mays. A phylogenetic tree for angiosperms based on nia exon sequences was more consistent with Cronquist's classification system than any other systems. Although nucleotide lengths of introns among taxa were very variable, its order and homology between them was well correponded to the exon tree. Also codon usage of nia genes at the third base pair was extremely biased toward G and C especially in monocotyledons up to 96% compare to dicotyledons in which varied from 40% to 66%.

Key words: angiosperms, molecular evolution, nia, nitrate reductase gene