how to add NOP in nios?

One might ask why you need a 'nop' - the problems are usually making the nios run fast, not slow!

The assembler will convert 'nop' to 'add r0,r0,r0' - which isn't a very good 'nop' since it changes the flags!

A better option might be one of the branch instructions.

'br . + 4' (0x6) is always two clocks.

'bne r0,r0,anywhere (0xxxxx << 6 | 0x1e) is 1 clock if correctly predicted (likely if backwards).