PCR is a powerful tool for the amplification of genetic sequences. It has been widely applied in molecular biology. It is generally used to amplify short segments (several hundreds basepairs to several kilobasepairs). It is difficult to amplify a long DNA segment. Based on the sequenced genes, it is known that most intact genes are very long. And intact gene is very important for the gene to express specially and effectively. Long PCR is a very useful tool to amplify intact genes for constructing special expression vectors. We have tried several chemicals to optimize long PCR system and found betaine was the best. Betaine, as an amino acid analogue with small tetraalkylammonium ions, could remarkably improve the amplification of long targets from the plant genome. The suitable concentration of betaine was between 1.0 mol/L and 2.5mol/L. We could effectively amplify a 9 kb DNA segment from maize genome DNA and a 16 kb DNA segment from plasmid. It was shown that different primers and different targets (different GC content) needed different concentrations of betaine. Betaine can reduce or eliminate non-special amplification. In the meantime we tried other additive chemicals, such as DMSO, glycerin, formamide. They were no notable results in long PCR.