display categories and all sub-categories

I have two top level categories - 1 and 2. The default setting if click 1 will show sub-categories 1.1 1.2 1.3...... . But I want to show all sub-categories in main page without click 1 or 2 . how can I do it, can I do it in layout setting?

1-----1.1
-----1.2
-----1.3
-----1.4
-----1.5
2-----2.1
-----2.2
-----2.3
-----2.4

What you want is known as "expanded categories", and there are mods available in Plugins to give the effect. One of them also displays products, which may not be desirable for you. One very reliable one is fragfutter's Expanded Categories Tree, available through a link from his forum thread to his website, or as an option (potentially standalone) bundled in Categories Dressing in Plugins.

is fragfutter's Expanded Categories Tree will work on zen cart 1.5.1, I read some post in this forum fragfutter's Expanded Categorie will work on 1.3x, I m not sure will work on 1.5.x or not, thank you

Yes, that mod (and Categories Dressing) will work on any version of Zen Cart since about v1.3.5 or so.

I would like to download fragfutter's Expanded Categories Tree from his forum thread to his website, but the link is not working anymore, is there other way I can download it, thank you

Fragfutter's Uncollapsed Categories Tree is still at his site, but you have to look very carefully to find it (page 2 of the "Software" category). Here is a direct link to the correct location (as of February 2, 2014):
http://www.held-im-ruhestand.de/software/zen-cart-categories

Alternately, the mod files are available bundled as an option in Categories Dressing in Plugins here.