OBJECTIVE： To investigate in vitro antibacterial activity of the extracts from the root， stem and leaves of Psychotria rubra， and to investigate its mechanism of antibacterial effects. METHODS： 95％ ethanol extracts from the root， stem and leaves of P. rubra were used to prepare solution with mass concentration of 100 mg/mL （calculated by extract）. Using as penicillin sodium （0.5 mg/mL） and streptomycin sulfate （128 mg/mL） as positive control， plat stiletto method was used to determine antibacterial effects of the extracts from different parts of Staphylococcus aureus， Escherichia coli， Pseudomonas aeruginosa， Bacillus subtilis， Micrococcus luteus and Enterococcus faecalis. Using systematic solvent extraction method， the petroleum ether， ethyl acetate and n-butyl alcohol were used to extract antibacterial activity parts from P. rubra as ad to obtain the extracts of corresponding polar parts. After preparing 100 mg/mL drug solution （calculated by extract）， antibacterial effects of above 6 kinds of bacteria were investigated. The micro-broth dilution method and agar culture medium plate method were used to determine minimum inhibitory concentration （MIC） and minimum bactericidal concentration （MBC）， and screen polar parts with bacteriostasis and drug-sensitive strains. Under the concentration of 0.5MIC and MIC， the growth curves of sensitive bacteria were drawn （treated for 36 h， every 4 h）； the contents of alkaline phosphatase （AKP） and soluble protein were detected in suspension （treated for 10 h， every 2 h）； bouillon culture medium instead of the extract as blank control. RESULTS： 100 mg/mL ethanol extracts from the root， stem and leaves of P. rubra showed good antibacterial effect to above 6 kinds of bacteria， in descending order as stem＞leaves＞root. Among different polar parts from the stem of P. rubra， antibacterial effect of the ethyl acetate extract was best， especially for the S. aureus， the diameter of inhibition zone reached （38.93±0.12） mm， and both MIC and MBC were 0.39 mg/mL. The ethyl acetate extract could significantly inhibit the proliferation of S. aureus， and its alkaline phosphatase and protein content in suspension were increased significantly compared with blank control （P＜0.05）. CONCLUSIONS： The stem from P. rubra is effective antibacterial parts， and exhibit good antibacterial activity to 6 kinds of common pathogens. The ethyl acetate extract from the stem of P. rubra shows strongest antibacterial effect on S. aureus， and could gradually destroy the integrity of cell wall and cell membrane.