I usually programme by C++ ,sometimes I also use C . I want to kown In a program ,C/C++ can mix together perfectly?

i think so...
for example i use // comments in C programms and works perfectly, however is C++ comments.
If the compiler supports both languages, why not? (gcc for example)

Most C code can be compiled as C++.

http://en.wikipedia.org/wiki/C%2B%2B#With_C

Yes! Although C is structure oriented and C++ is object oriented, to some extent it can be mixed. The basic functions other than the i/o, are same i.e. have the same format like for loop, if-then-else,while, user defined functions,arrays ans many more.

Yea sure you can use mix C and C++ together because C++ compiler does recognize C language.