I had a similar problem recently using a binary compiled for the wrong architecture. It's more likely that you have a path issues (as others have suggested), but this is worth checking as well. For example, when I download the wrong version of the SRA toolkit to my system, I get the same error when I try to execute the binaries.

standage@gremlin4:~$ wget http://trace.ncbi.nlm.nih.gov/Traces/sra/static/sratoolkit.2.1.6-ubuntu32.tar.gz
standage@gremlin4:~$ tar xzf sratoolkit.2.1.6-ubuntu32.tar.gz
standage@gremlin4:~$ cd sratoolkit.2.1.6-ubuntu32/
standage@gremlin4:~/sratoolkit.2.1.6-ubuntu32$ ls -l fastq-dump.2.1.6
-rwxr-xr-x 1 standage standage 1633589 2011-09-12 11:11 fastq-dump.2.1.6
standage@gremlin4:~/sratoolkit.2.1.6-ubuntu32$ ./fastq-dump.2.1.6 
-bash: ./fastq-dump.2.1.6: No such file or directory

If I compile from the source code, the program executes successfully. If your path is correct, I would check whether the binaries were compiled for the correct architecture.

"No such file or directory" typically means that the executable (blastn in this case) was not found in the directory from which you tried to run it. So it's likely that $PATH does not include /home/abc/ncbi-blast-2.2.25+/bin/blastn.

Try this. Edit a file named .profile in your home directory and add these lines:

if [ -d "/home/abc/ncbi-blast-2.2.25+/bin" ] ; then
    PATH="/home/abc/ncbi-blast-2.2.25+/bin:$PATH"
fi

Note that the path points to the BLAST bin directory, not the blastn executable.

Then in the terminal run:

source ~/.profile

(only this time - the file will be read next time you log in.)

Then try blastn with no arguments (in the same terminal) to check that it is found.

EDIT: based on your response, another possibility is that blastn is not executable. What does:

ls -lrt /home/abc/ncbi-blast-2.2.25+/bin/blastn

show you?

I h've set everything.echo $PATH command giving me this output /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/home/abc/ncbi-blast-2.2.25+/bin..

And when i am running blastn command wothout any comment or with complete path /home/abc/ncbi-blast-2.2.25+/bin/blastn -help In both case it showing me the same error that bash: /home/abc/ncbi-blast-2.2.25+/bin/blastn: No such file or directory...

I am doing the same thing in other system and its working fine...But not in my system...

Thanks Manju Rawat.