Let’s say there are two .pst files you will merge, and one is opening in Microsoft Outlook, while the other has not been opening, you can merge them with following steps:

Step 1: Open the Import and Export Wizard dialog box with one of following methods:

1. In Outlook 2007, click the File > Import and Export;
2. In Outlook 2010, Click the File > Open > Import;
3. In Outlook 2013, click the File > Open & Export > Import/Export;

Step 2: In the coming Import and Export Wizard dialog box, click to select the Import from another program or file, and then click the Next button.

Step 3: In the Import a File dialog box, click to select the Outlook Data File (.pst), and then click the Next button.

Step 4: In the Import Outlook Data File dialog box, check the option of Do not import duplicates, and then click the Browse button.

Step 5: In the coming Open Outlook Data Files dialog box, find out and select the .pst file you don’t open in Outlook, and then click the Open button.

Step 6: Click the Next button in the Import Outlook Data File dialog box.

Step 7: In the Second Import Outlook Data File dialog box, check the option of Import items into the same folder in box, and then clicking the box below and specify the .pst file that you have opened in Outlook, and click the Finish button.

Step 8: Then a Microsoft Outlook dialog box pops up to show you the importing progress. And it will be closed automatically when importing finishes.

If the two .pst files you will merge have not been opened in Microsoft Outlook, you have to open one of them before merging.

You can open a .pst file in Outlook with following methods:

1. In Outlook 2007, click the File > Open > Outlook Data File
2. In Outlook 2010, click the File > Open > Open Outlook Data File
3. In Outlook 2013, click the File > Open & Export > Open Outlook Data File

And then in the Open Outlook Data File dialog box, find out and select one of both .pst files, and click the Open button.

Now you can follow the same steps we introduced in the first situation to merge the two .pst files.

If both of .pst files you will merge have been opened in Microsoft Outlook already, you have to close one of the .pst files before merging.

Right click the .pst file you will close in the Navigation Pane, and then select the Close “Pst File Name” from the right-clicking menu. See screen shot below. Note: the “Pst File Name” will show as your pst file name.

Then you can follow the same steps we introduced in the first situation to merge the two .pst files.