Sliding chain over the cassette Today I have replaced my chain with a kmc z8s. The rear cassette is Shimano acera 8 speed , and is not worn yet , so everything is OK with the cassette. But the problem is that the chain doesn't fit the cassette properly ,when I apply more power on the pedals the  chain starts sliding over the cassette (The lenght of the chain is OK too). What is the solution of this problem ?
The cassette really is worn.  It takes something of a trained eye to spot a too-worn cassette, and often when you switch to a new chain the cassette will skip worse than it did with the old, worn chain.
The chain is the wrong one for the cassette.  Chain/cassette compatibility is an area where there are no nice general rules -- you have to make sure that the chain you get is compatible with your specific cassette. This includes having the correct speed pairing (e.g. 8-speed chain for 8-speed cassette).
The chain was cut to the wrong length.  A chain is always delivered with 8-10 inches of extra chain.  You must shorten it to the appropriate length, using a "chain breaker".  Some people just wrap the chain around the largest sprockets and eyeball it, but if you haven't developed that skill the best thing to do is to lay old and new chain side-by-side and carefully match up links along the length so that there are the same number of links in the new chain as the old.  (Note that if you just match length the new chain will likely end up a couple of inches too long.)
The chain was threaded through the rear derailer incorrectly.  It's kind of hard to mess up on the front, but the threading through the rear derailer is not intuitive, and it's possible to muck it up.
The derailer is out of adjustment or has been bent somehow.