Hi,
I do gym from 1 yr, but there is no improvement in arms. One of my friend said that training biceps and triceps at same time will increase the arms size.is it true??

They are separate muscles. You can train them separately or together, it won't make any difference.

Actually, there was an interesting study where two groups of men trained arms, but one group also trained legs. The group that trained legs as well as arms got bigger arms than the group that trained arms only. The increased testosterone and growth hormone from the leg training helped to increase arm size.

Personally, I would stop worrying about training individual arm muscles and concentrate on big compound lifts like bentover barbell row, pull-up, dips, bench press, and you'll find they hit your arms much harder than endless biceps curls.

Your tri-cep is 2 thirds of your upper arm in comparison to 1 third for your bi-cep. If you want your arms to be bigger you will have to hit your tris more often or harder.

Training all over as well will make your arms bigger, compound lifts and movements with high weight and proper form will build size due to the release of GH.

Also you could try training your back and tris one day and the next day do your chest and bis - a buddy of mine has been trying this routine and it seems to have helped him out quiet a bit.

I would recommend hitting your tri''s the same day as your chest since you are already warming up the muscles when you bench. Work the tris last because if you work them first you won't have anything left for benching so always hit the largest muscle group first working down towards the smaller muscle groups. You also don't want to overtrain them working them no more then twice a week. I usually work my chest and tri's on Monday then follow up again on Friday along with my Biceps. Make sure you are hitting all 3 heads of your tricpes mixing up your workout with different exercises, weights, reps, sets or rest times should never be avoided. So many people strive for 3 reps of 10 and never vary. What I do is have a diary of my workout and each time I strive for one additional rep or even more weight maybe go for 5 sets instead of 3 once in a while, even try adding if its just a couple pounds, then I total the weight by reps for weight lifted so If say I lift 100 pounds by 10 reps by 3 sets would equal 3000 pounds, I try to beat that any way I can the next week, this way I know I am progressing and you would be amazed that is such a motivator when you know what it takes to beat it even it ifs only 100 pounds over 3 sets or 5 sets its an improvement and you will see growth, that mentally really makes a difference..
Because if you don't know from one week to another what you are lifting by a diary you really won't know if you are pushing yourself.
I hope this provides some help. Also go to the library and pick up some muscle and fitness magazines as they usually carry back issues that are great reads and free, they have tremendous information that is very useful for novice lifters as well as advanced lifters as there is a lot of information that even the pros conflict about so lot of it is personal preference... But the one thing is GET A DIARY and record your workouts...
Bill