There are many methods to truncate all the tables in a database.

Part I of my post Truncate all tables shows executing the concatenated truncate table statements. This is other method

Set NoCount ON

Declare @tableName varchar(200)
set @tableName=''

While exists
( 
--Find all child tables and those which have no relations

select 
	T.table_name 
from 
	INFORMATION_SCHEMA.TABLES T
    left outer join INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC
    on T.table_name=TC.table_name     
where 
	(TC.constraint_Type ='Foreign Key'or TC.constraint_Type is NULL) and
    T.table_name not in ('dtproperties','sysconstraints','syssegments')and     
    Table_type='BASE TABLE' and T.table_name =@TableName)

Begin


select 
	@tableName=min(T.table_name)
from 
	INFORMATION_SCHEMA.TABLES T
    left outer join INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC
    on T.table_name=TC.table_name     
where 
	(TC.constraint_Type ='Foreign Key'or TC.constraint_Type is NULL) and
    T.table_name not in ('dtproperties','sysconstraints','syssegments')and     
    Table_type='BASE TABLE' and T.table_name =@TableName

        --Truncate the table
     Exec('Truncate table '+@tablename) End set @TableName=''

While exists
( --Find all Parent tables 

select 
	T.table_name 
from 
	INFORMATION_SCHEMA.TABLES T
    left outer join INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC
    on T.table_name=TC.table_name     
where 
	TC.constraint_Type ='Primary Key'and T.table_name <>'dtproperties' and
	Table_type='BASE TABLE' and T.table_name =@TableName 
	)
    Begin


Select 
	@tableName=min(T.table_name) 
from 
	INFORMATION_SCHEMA.TABLES T
    left outer join INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC 
    on T.table_name=TC.table_name     
where 
	TC.constraint_Type ='Primary Key' and T.table_name <>'dtproperties' and     
	Table_type='BASE TABLE' and T.table_name =@TableName      
	
    --Delete the table
    Exec('Delete from '+ @tableName)      --Reset identity column 

    If exists    (         select * from information_schema.columns         where COLUMNPROPERTY(OBJECT_ID(QUOTENAME(table_schema)+'.'+        QUOTENAME(@tableName)), column_name,'IsIdentity')=1    ) 

    DBCC CHECKIDENT (@tableName, RESEED, 1)

End
Set NoCount Off