Hi Ritika

If you are trying to import the workitems csv in the IBM RTC eclipse client, you need to change the maximum heap size.
Open the eclipse.ini file and change the Xmx parameter value to 1024m.
Location of eclipse.ini: <<RTC installation location>>\eclipse.ini
search for value:
-vmargs
-Xmx512m

Change the value to below:
-Xmx1024m

-Restart the IBM RTC eclipse client.

If this value still gives error, try changing to -Xmx1536m.
You can also track how much heap  is getting occupied in the IBM RTC eclipse client:
https://www.ibm.com/developerworks/community/blogs/nfrsblog/entry/ibm_rtc_eclipse_client_and_monitoring_the_heap_space

Alternatively, if you want to minimize the investigations,split the excel file and import the different excel one by one.

Let me know if this helps.

Regards,
Rohit