I just installed 4 new doug fir (green wood) 6x6 support posts. They are starting to crack. should I change them out with some other type of wood, or get new posts of the same type or are they alright and I should just ignore the developing cracks??

Are they cracking, or are they checking? Checking is normal, cracking is not.

Douglas fir is not "green wood"...what do you mean by "green wood"?

Checking? not sure what the difference between cracking and checking is. Anyway, what I ment be "Green Wood" was not kiln dried.

Aha, if it was that kind of "green" wood, then checking is very much a probability. Checking is a surface split which doesn't go very deep, and does not affect the structural capacity of the member. Cracking goes all the way through the member and usually represents a failure of some sort. Generally, 6x6 posts in a basement are probably severe overkill, and are very unlikely to fail.

If the wood was surfaced green, it will very likely check. If it was surfaced dry, it's less likely but still somewhat common. It's a natural effect in wood, caused by the differences in drying rates between the inner and outer rings of the log. The outer rings of wood dry quicker and therefore shrink, while the inner rings remain green, so something has to give, and you get what appears to be a crack, but isn't, it's a check. That's very likely what you see.

Ok, I'm satisfied with that answer. I went down and tried sticking a piece of paper into the crack/checking. It only went in less that .5". That's good to know because it was a lot of work to put those things in and I didn't like the thought of taking them out again. Now it's on to the next basement block. Tring to get it framed. Thanks!!!!

I have a similar problem with trees as supports in my basement and after having heat this winter they are spliting. My game plan is to replace them with metal poles/supports with max of 20,000 pds and space them every 8 or 10 feet. I have a small one bedroom second floor w/o bath in this cottage. Sound reasonable???

Sure does. You need to provide sufficent temporary support so you can remove the split posts. Once done you need to dig a foundation footing where each replacement post will sit. This must be approved by the local building department as they have requirements on how big and deep they will need to be.
Once the cement dries you can place cement filled steel posts in place of the old wood ones.

As far as the splitting of the posts. This really does not effect the vertical load on them. Unless they are splintering. Its very common to see wood columns that are aged and split. If your concerned and do not have the time right now to do the job properly. You can always wrap the posts with steel wire to prevent the posts from splitting more.