How to show related contacts under account in a Salesforce Visualforce page?

Hello RadhaKrishna

Please Try below Code

Controller -

public class actocons
{
public list<account> acct{set;get;}
public actocons()
{
acct=[select id,name,(select id,lastname from contacts)from account limit 5];
}
}

VisualForce Page -

<apex:page controller="actocons" tabStyle="Account">
<apex:pageBlock >
<apex:repeat value="{!acct}" var="a">
<apex:pageBlockSection id="section1" title="{!a.name}" onclick="document.getElementById('{!$Component.section1}').childNodes[0].childNodes[0].click();">
<script>twistSection(document.getElementById("{!$Component.section1}").childNodes[0].childNodes[0]);document.getElementById("{!$Component.section1}").childNodes[0].style.cssText = "cursor:pointer;" </script>
<apex:outputField value="{!a.Name}"/>
<apex:pageBlockTable value="{!a.Contacts}" var="c" id="j1">
<apex:column value="{!c.id}" />
<apex:column value="{!c.lastname}"/>
</apex:pageBlockTable>
</apex:pageBlockSection>
</apex:repeat>
</apex:pageBlock>
<script>
twistSection = (function() {
var cached_function = twistSection;

return function(twisty, sectionId) {
cached_function.apply(this, arguments); // use .apply() to call the original twistSection function provided by Salesforce
if (event.stopPropagation) {
event.stopPropagation();
} else {
event.cancelBubble = true;
}
};
}());

</script>
</apex:page>