You can enter Tibet by China side, there are flights to Lhasa form different cities of China or you may enter by train to Lhasa. Air China weekly operates flight from KATHMANDU to (LHASA & CHENGDU) every Saturday Jan to March then every Saturday & Tuesday March to June. From July - October every day depending upon the weather conditions. The 55 minutes flight offers spectacular views of the Himalayas and the southern Tibetan plateau.
An approach that has great appeal for hardy and adventurous travelers is to enter or leave Tibet by road. Nominally, organized tours must be arranged before you enter Tibet. Moreover to get into Tibet from outside Tibet from Chengdu or Kathmandu you have to book a so - called "Tour".
- Nepal Tibet Tours & Treks