Most people do it as a group, but concerning about weather and transportation, it is very hard to do that. But theoretically, it could be done.

Dear Ms. Grace,

Thanks for your reply.

I would just like to clarify if a sole traveller can travel from Nepal to Tibet alone and pass the immigration, as my friends told me that Tibet immigration don't allow single entry.

hi,
im an oversea student in china now, from singapore. im sorry to tell u that its impossible to travel along in tibet if u are a foreigner. it requires ur tibet travel permit which u must book an agency to help u to get it from the local govern,(u can not get it by urself). i have been there, just with the help of an agency. since i dont like group travel, so i just book an private trip with an agency. u need to hire at least a guide to acompany u all ur trip, but dont worry the guide cannt be ur shadowing nanny and u have abosulte freedom to decide ur itinerary and ur guide could be only ur director or driver.
good luck