http://codex.wordpress.org/Template_Tags/wp_dropdown_categories

You can use the code snip they’ve provided

<script type="text/javascript"><!--
var dropdown = document.getElementById("cat");
function onCatChange() {
if ( dropdown.options[dropdown.selectedIndex].value > 0 ) {
location.href = " ?>/?cat="+dropdown.options[dropdown.selectedIndex].value;
}
}
dropdown.onchange = onCatChange;
--></script>
</li>

Parameters

show_option_all 

(string) Causes the HTML for the dropdown to allow you to select All of the categories.

show_option_none 

(string) Causes the HTML for the dropdown to allow you to select NONE of the categories.

orderby 

(string) Key to sort options by. Valid values:

• 'ID' (Default)
• 'name'

order 

(string) Sort order for options. Valid values:

• 'ASC' (Default)
• 'DESC'

show_last_update 

(boolean) Sets whether to display the date of the last post in each category. Valid values:

• 1 (True)
• 0 (False – Default)

show_count 

(boolean) Sets whether to display a count of posts in each category. Valid values:

• 1 (True)
• 0 (False – Default)

hide_empty 

(boolean) Sets whether to hide (not display) categories with no posts. Valid values:

• 1 (True – Default)
• 0 (False)

child_of 

(integer) Only display categories that are children of the category identified by its ID. There is no default for this parameter.

exclude 

(string) Comma separated list of category IDs to exclude. For example, ‘exclude=4,12′ means category IDs 4 and 12 will NOT be displayed/echoed or returned. Defaults to exclude nothing.

echo 

(boolean) Display bookmarks (TRUE) or return them for use by PHP (FALSE). Defaults to TRUE.

• 1 (True – default)
• 0 (False)

selected 

(integer) Category ID of the category to be ’selected’ or presented in the display box. Defaults to no category selected.

hierarchical 

(boolean) Display categories in hierarchical fashion (child categories show indented). Defaults to FALSE.

• 1 (True)
• 0 (False – Default)

name 

(string) Name assigned to the dropdown form. Defaults to ‘cat’.

class 

(string) Class assinged to the dropdown form. Defaults to ‘postform’.

depth 

(integer) This parameter controls how many levels in the hierarchy of Categories are to be included in the list of Categories. The default value is 0 (display all Categories and their children). This parameter added at Version 2.5

• 0 – All Categories and child Categories (Default).
• -1 – All Categories displayed in flat (no indent) form (overrides hierarchical).
• 1 – Show only top level Categories

n – Value of n (some number) specifies the depth (or level) to descend in displaying Categories

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