Protein tyrosine phosphatase 1B (PTP1B) is emerging as a potential therapeutic target for the treatment of type-2 diabetes and obesity. To search for new types of PTP1B inhibitors, we have undertaken in vitro enzyme assay for some anthraquinones and stilbenes isolated from plants. Of the anthraquinones tested, physcion (1), 1-O-methylemodin (2), and emodin (3) showed high activities, with $IC_{50}$ values of 7.6, 7.0, and $3.8{mu}g/mL$, respectively, while the anthraquinone glycosides, physcion-8-O-${eta}$-D-glucopyranoside (4) and emodin-8- O-${eta}$-D-glucopyranoside (5), were less active than their aglycones. All the stilbenens (6 - 15) slightly inhibited PTP1B activity at high concentration of $30{mu}g/mL$. Our findings suggest that the hypoglycemic effect of anthraquinones may be associated with their PTP1B inhibitory activity.