Dynamic　spectrum　access　is　one　of　the　most　important　premises　of　spectrum　reuse　based　on　cognitive　radio　technologies,　which　are　considered　to　be　the　best　way　to　alleviate　the　controversy　on　spectrum　scarcity　and　low　efficiency.　However,　most　of　previous　work　focuses　on　the　increasing　of　system　throughput,　ignoring　the　QoS　requirement　of　secondary　users.　In　this　paper,　dynamic　spectrum　access　with　QoS　provisioning　is　studied.　We　adopt　discrete-time　Markov　chain　to　analyze　and　model　the　spectrum　usage　in　time-slotted　cognitive　radio　networks.　Furthermore,　three　admission　control　schemes　are　proposed　to　minimize　the　forced　termination　probability　of　secondary　users.　Simulation　results　show　that　our　proposed　schemes　can　significantly　improve　the　forced　termination　probability　of　secondary　users,　though　slightly　increase　the　blocking　probability.