i wanna know how u guys know if there is a player need to be substituted in both games. How do u know if he is fatigued. please...

Go into team management, then substitutions.
There is a column there which I think is under the heading fitness. The number in this column will be 100% at the start of the game and will gradually decrease as you go.
It will decrease faster if you use the sprint button too often.

In FIFA 99 a player with Stamina of less then 8 would be usually tired after one half of play and AI would move in to substitute for him(if the team was cpu controlled of course).

In FIFA 2000 it doesnt matter what your stamina is, it seems to have zero bearing on whether players will get tired. As experiment I lowered stamina to 1 for certain players, while upping it to 7 for others, and there seemed to be absolutely no correlation as far as players getting tired based on low stamina rating. Also the AI will never ever!!! substitute a tired player on the opposing team. How could EA just totally screw up this part of the game is beyond me, they broke everything that was good about 99 that didnt need fixing!

Fatigue is indeed insignificant in the fifa200.

If you were to play a 4, 6 mins game or longer, the fatigue has no significant effect at all!

But I think you can see how tired the player is by looking at the visual indicator (the circle highlighting the player). By looking at how filled the circle is, u can determine the player's level of tiredness.

But who cares!!! Fatigue is insignificant in fifa2000.