  We study the thermodynamic stabilities of uncharged and charged black holes
surrounded by quintessence (BHQ) by means of effective thermodynamic
quantities. When the state parameter of quintessence $\omega_q$ is
appropriately chosen, the structures of BHQ are something like that of black
holes in de Sitter space. Constructing the effective first law of
thermodynamics in two different ways, we can derive the effective thermodynamic
quantities of BHQ. Especially, these effective thermodynamic quantities also
satisfy Smarr-like formulae. It is found that the uncharged BHQ is always
thermodynamically unstable due to negative heat capacity, while for the charged
BHQ there are phase transitions of the second order. We also show that there is
a great deal of difference on the thermodynamic properties and critical
behaviors of BHQ between the two ways we employed.
