Ethanol crude extract of Piper betel Linn. (Piperaceae) leaves were tested under in vitro for antifungal activity against plant pathogenic fungi in economic crops. It exhibited 100 percent inhibition against all tested plant pathogenic fungi at a concentration of 1 % using the dilution method whereas inhibition was evident at 10% when tested by the diffusion method. The results suggested that crude extract exhibited fungistasis activity by inhibiting mycelial growth, and fungicidal activity by inhibiting spore germination. Fractionation of crude extract by column chromatography eluted with 51 solvent combination systems of increased polarity yielded 51 micro fractions which were tested for inhibition activity against plant pathogenic fungi in vitro. The micro fractions F4, F5, F7 and F8 showed antifungal activity and the TLC chromatogram indicated the common compounds were contained in these four fractions. Further re-fractionation of the most active fractions, F7 and F8 were eluted with mixture of petroleum ether and chloroform and tested for antifungal activity. The 21 submicro fractions of PE: CHCl3 at ratio of 40:60 to 32:68 exhibited antifungal activity with the same results as in the former experiment. The TLC chromatograms indicated the presence of the like compounds and it is suggested that the active compounds might be contained in these sub micro fractions of PE: CHCl3. The dosage response curve indicated a positive linear relationship between concentrations and the level of antifungal activity. NMR (Nuclear Magnetic Resonance) spectroscopy was conducted to elucidate the chemical structure of the active compound contained in the fractions F7 and F8 which was identified as hydroxychavicol. The study concluded that hydroxychavicol in the crude extract from P. betel leaves is an active constituent against plant pathogenic fungi.