I am simulating a model including a few chemical reactions.

When I initialize the solver or set boundary conditions, I can see often only 5 speices, although in "Materials" panel I have defined 6.

Really I do not initialize or set the value of this missing component. But, isn't it a serious problem? I want to know, whether it affects the result.

I thank you all, who have given me advices about the problem with UDFs, and how to run the Fluent on Linux/Unix server, usw.

Finally I can run my UDF program, although there are still many problems.

I am so happy and thank for your help. ^_________________^

When you use the species model, the last species listed in the model is assumed to the "balance" such that if your mass fractions in your BC for the first 5 (n-1) species don't add up to 1, the missing last species is assumed to be the rest as to make the total for 6 (n) species to be 1.
Note that if you want a different species to be the balacne, you have to add that one last in the mixture setup. Generally, while I don't think it affects the solution, choosing the species with the most inert character in the overall reaction scheme or is of the least importance in the resulting analysis as the balance is the best choice.
Also note that while you can't specify that component in BC's, you will be able to plot concentrations and the like of that species with post-processing.