Currently I'm working in an International law firm. I plan to apply LL.M 2008 but I don't want my partner know now, so I ask one of my supervisor(associate) to write the recommandation letter for me. Will it differ a lot from a rl letter signed by partner?

Anybody can help? : )

you usually ask a professor who knows of your academic work to write you a recommendation for LLM; otherwise if you have had clerkship experience, you can ask your judge to write you a recommendation. if the program you are applying to requires a work supervisor to write your recommendation letter, i think you should ask your partner that you do most of your work with to write you one. a recommendation from a partner is definitely more impressive than one from an associate, who is basically your peer. also, showing to the partner that you want to improve your qualifications should be taken as a good thing (which i assume you will fund yourself or through a scholarship that you will apply for yourself), since whatever learning you acquire from your LLM should be able to help you become a better performer in your firm when you come back with your LLM. given this, it's a good idea to just go on leave from work when taking your LLM, on account of possible difficulties in finding post-LLM opportunities (that's what i am doing right now). of course, if we find something better, then that's the time to resign. :)

Most law schools prefer a letter from someone who taught you. Approaching a Law School Faculty, who will have no adverse interest against you, would still be better reference. The recommendations letters are very important in USA Law Schools and should answer specifically what is asked for in the cover letter provided by the schools as a part of the application form.