Human multiple myeloma is a presently incurable hematological malignancy and novel biologically based therapies are urgently needed. Triptolide (TPL) is a purified diterpenod isolated from the Chinese herb, Tripterygium wilfordii Hook. F that has shown antitumor activities in various cancer cell types. But its activity in Dex-resistant multiple myeloma cell lines and the main upstream signaling pathway has not been reported. Here we show that TPL induces apoptosis in dexamethasone-sensitive (MM.1S) and dexamethasone-resistant (MM.1R) cells, most importantly its main upstream signaling pathway is through the PI3k/Akt/NF-kappaB pathway and is also associated with MAPK pathway, via mitochondrial apoptotic signaling and is also associated with the caspase and Bcl-2 family members. Moreover, TPL was able to enhance the activities of dexamethasone or bortezomib/PS-341 in multiple myeloma cell lines. Collectively, these findings provide the framework for a clinical evaluation of TPL, either alone or in combination with dexamethasone or bortezomib/PS-341, to overcome drug resistance and improve outcome for patients with this universally fatal hematological malignancy.