Q:

Using Capital letter for a word in a sentence I want to put emphasis on?

I sometimes tend to use the Capital letter for some words in which I would want to give emphasis on.
For example

Please deliver the Task on Time.

I am trying to catch the users attention on how important the task and time is.
Is this correct usage?

A:

Using capital letters within sentences makes a writer seem uneducated and ignorant. One can use typographical effects such as italic, bold, underlining, etc, but even these should be used sparingly. 

...use bold and italic as lit­tle as pos­si­ble.

Bold or italic (Butterick's Practical Typography)
The Modern Language Association agrees (in my opinion this applies to bold as well):

The MLA style discourages the use of italics in academic prose to
  emphasize or point, because they are unnecessary—most often, the
  unadorned words do the job without typographic assistance. And if they
  don’t, then rewording is often the best solution. This policy is a
  matter of stylistic convention, not grammar.

Italics for emphasis (MLA Style center)

