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$I$ is commenly used as a notation of identity matrix. I am wondering is there any notation else for identity matrix?

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    $\text{Id}$ is also common. – 2017-01-06
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    I have seen the use of $I_d$ to denote a $d\times d$ identity matrix. As an added comment to JimmyK4542's, you may also see the more explicit notation $\operatorname{id}_V$ where $V$ is the relevant vector space. – 2017-01-06
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    In some contexts, I have also seen a doublestrike $1$, similar to the difference between $N$ and $\mathbb N$, in order to emphasize that it is the compositional identity. The fact that I am unable to type it in mathjax right away however should imply something about how uncommon that notation is though. – 2017-01-06
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    I use \mathds{1} to get the double strike 1 (aaah so lovely) but the dsfont package isn't a default. Picked it up from some random lecture notes from someone at EPFL and liked it. – 2017-01-06
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    More rarely now, but at some point $E$ was used for the identity. – 2017-01-06
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    @copper.hat: $E$ is sometimes used in German (*Einheitsmatrix*), and also in my native Swedish (*enhetsmatris*). – 2017-01-06
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    @HansLundmark: Thanks, that makes sense! – 2017-01-06
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    @copper.hat that is cool~ definitely what i want :) – 2017-01-06

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From Wikipedia:

It is denoted by $I_n$, or simply by $I$ if the size is immaterial or can be trivially determined by the context. (In some fields, such as quantum mechanics, the identity matrix is denoted by a boldface one, 1; otherwise it is identical to $I$.)

It can also be written using the Kronecker delta notation:

$$(I_{n})_{ij}=\delta _{ij}.$$ Hope it helps.

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    Thank you, @Alex Mathers for the edit of yours. – 2017-01-06
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$I_n$

$I_m$

$I_n = diag(1,1,1,1\cdots,1)$

$(I_n)_{ij} =\delta_{ij}$