If you are entering Tibet from Kathmandu, you should get your Chinese visa from the Chinese Embassy in Kathmandu (regardless of whether you already have a Chinese visa issued in your country). Please note that if you have already got a Chinese visa at your home country, it will be cancelled and fresh visa will be issued in Kathmandu. So do not apply for a Chinese visa until you reach to Kathmandu and avoid waste of money (in the case you enter Tibet from Nepal).
However if you are entering Tibet form mainland China or Honking, get your Chinese visa in advance. You are required to hold a valid Chinese visa (issued by a Chinese Embassy and stamped in your individual passport), as well as a valid passport to enter Tibet (at least six months valid). When applying for a Chinese visa at your home country, it is better not mentioning that you intend to visit Tibet.
In order to get Chinese visa in Kathmandu, please send us your clear passport copy and other details like your profession to us at least 15 days in advance. We will get Tibet Travel Permit (TTB permit or commonly called Invitation letter to travel to Tibet from the Tibet Travel Bureau – TTB). Once you arrive to Kathmandu, we apply for your Chinese visa along with your original passport and TTB permit. You will get a group visa (it will be not stamped in your individual passport but will be in a sheet of paper).
Very important: The visa days in the Chinese Embassy in Kathmandu is Monday, Wednesday and Friday ( and visa can be collected the same day upon payment of urgent fees – USD 114 for European passports and USD 198 for US passports ) . In order not to waste your time in Kathmandu, make it such that you arrive to Kathmandu either Sunday or Tuesday or Thursday so that you can get visa the next day and fly to Tibet the day after. (Except in main season, the flights to Lhasa from Kathmandu are generally operated on Tuesday, Thursday and Saturday mornings and almost always get packed)