just a doubt

From the FAQ:

#How do ranked leagues work?

There are five divisions (V through I, with I being the highest) and six tiers (Bronze, Silver, Gold, Platinum, Diamond and Challenger).

After you play your initial placement matches, the system will place you in a specific tier and division based on your results. From there, you’ll have to climb the ladder by winning matches and improving! Win ranked games and you’ll gain League Points (LP), while losing in ranked loses you LP. If you’re already at the bottom of your division, this may mean dropping to the previous division.

Each time you hit 100 LP within your division, you'll qualify for a promotion series best-of-three. Beat that and you’ll promote yourself to the next division. If you hit a promotion series when you’re in division I, then you'll qualify for a larger promotion series best-of-five. Beat that series and you’ll move on to the next tier!

You're briefly protected from falling to a lower tier after your promotion, but you can still fall back down to a lower tier if your skill level deteriorates rapidly and you find yourself playing at the skill level of a much lower-tiered player.

tl;dr yes, although you have a couple games where you're immune, after promotion.