Transmission

I am using transmission in UBUNTU 10.10. My net speed is 512KBps. When I use Downthem all (Addon in firefox web browser), I get downloading(Download + Upload) speed greater than 200KBps, sometimes above 300KBps. But at the same time in Transmission total download + upload speed is near 100KBps, that is download may be near 60 KBps... some time only for a second or a few seconds,in rare case, it reaches above 100KBps-150KBps download speed. Some time it also reached 220KBps. But only for a moment. All time its download speed is below 100KBps. the download speed limit is not on, and I tried alot. But I couldn't rectify it. Any one can help me.
{note: I got more than 100KBps download speed in my net connection using bittorrent software in windows xp OS}

i've found with transmission or any other bittorrent client that you should

1) make sure your listen ports are forwarded (open for incoming connections)

2) limit upload speed to about 70% of your maximum

there are other performance tweaks (like upload slots per torrent) but the above in my experience were the most important.

Thank You for your assistance ... I have tried both of your suggestions ... but unfortunately ..... still the download speed is not good.

You're comparing a Bittorrent client to an HTTP download extension for a browser? The two protocols aren't compatible.
DownloadThemAll grabs files sequentially and from a central server. The full speed of that download depends on the stress of the server and your download speed.
Bittorrent is distributed. It grabs the data from many peers at once. The speed depends on the swarm's ability to share. If you're on a new torrent in a public swarm, peers are less likely to share, and the speed suffers. A torrent with a well seeded swarm is more likely max out your connection. Some clients also have tit-for-tat algorithms. If you're not sharing to them, they're less likely to share to you. Don't limit your upload speed too low.