How we can disable the lookup input field on the Salesforce visualforce page?

Hi Mohit,
Apex:inputField does not have the attribute "disabled". You may have to do this through rendering or jQuery.

Below is the sample code for rendering :-
Sample Code:

Visualforce page:

<apex:page controller="sample">

<script type="text/javascript">

</script>

<apex:form >

<apex:pageMessages />

<apex:pageBlock >
<apex:pageBlockSection columns="2">
<apex:pageblockSectionItem >
<apex:outputLabel value="Metro"/>
</apex:pageblockSectionItem>
<apex:pageblockSectionItem >
<apex:inputCheckbox value="{!metro}">
<apex:actionSupport event="onchange" reRender="a" action="{!demo}"/>
</apex:inputCheckbox>
</apex:pageblockSectionItem>
<apex:pageblockSectionItem >
<apex:outputLabel value="City"/>
</apex:pageblockSectionItem>
<apex:pageblockSectionItem >
<apex:inputField value="{!city}" id="a" disabled="{!bool}"/>
</apex:pageblockSectionItem>
</apex:pageBlockSection>
</apex:pageBlock>

</apex:form>

</apex:page>

Apex Code:

public class sample
{
public Boolean metro {get;set;}
public Boolean city {get;set;}
public Boolean bool {get;set;}

public sample()
{

}

public void demo()
{
if(metro)
{
bool = true;
}
else
{
bool = false;
}
}

}