Import posts, pages, comments, custom fields, categories, tags and more from a WordPress export file.
If your exported file is very large, the import script may run into your host's configured memory limit for PHP.
A message like "Fatal error: Allowed memory size of 8388608 bytes exhausted" indicates that the script can't successfully import your XML file under the current PHP memory limit. If you have access to the php.ini file, you can manually increase the limit; if you do not (your WordPress installation is hosted on a shared server, for instance), you might have to break your exported XML file into several smaller pieces and run the import script one at a time.
For those with shared hosting, the best alternative may be to consult hosting support to determine the safest approach for running the import. A host may be willing to temporarily lift the memory limit and/or run the process directly from their end.
More Info: WordPress Codex: Importing Content
More Info: tp2wp.com/faq