How can I bring the late start date is same as project start date?

Probably you will find you are having negative float of one day, this should be your most critical path. Try reducing the duration of one of your most critical activities by one day.

Remember about job finish constraint defined at the project level and not at the activity level, this constraint seems to be overlooked /forgotten by many.

Another reason can be resource leveling, if doing resource leveling keep in mind that "traditional" Float computation is wrong, P6 provides you with a new definition called Remaining Float that is dependent on resource leveling but i am not sure it is the same as Resource Critical Float. As a matter of fact I have the suspicion it is not, otherwise Primavera would have used Resource Critical Float in their computation of "traditional" Float, as by definition both should be the same.

The following reference can provide you with some additional information about P6 dates.

http://ronwinterconsulting.com/Understanding_P6_Dates.pdf

Because I do not have P6 I could not investigate if Remaining Float as computed by P6 is the equivalent of Resource Critical Float or if it is another intermediate value, another of the many they provide without further explanation.

Ajith,

Check the constraints. If there is a late constraint then this might happen.

Also check for Mandatory Constraints.

Remove above constraints and re-schedule it. Let us know the late start date.